Plant-based transparent flame-retardant coating for ancient building protection and application and preparation method thereof

Abstract

The application relates to the technical field of fire-retardant coatings, in particular to a plant-based transparent fire-retardant coating for protecting ancient buildings, application and a preparation method, and aims to solve the technical problem that the existing transparent fire-retardant coating causes surface "red change" when applied to old and wet wood. The coating is characterized in that a multifunctional composite protection system is constructed, including a plant-based wood interface stabilizing primer, a plant-based transparent intumescent fire-retardant mid-coat paint and a plant-based weather-resistant finishing paint, and a wood pretreatment process is additionally provided. The core components are a plant-based interface stabilizer (containing a natural phenolic radical scavenger and a plant-based chelating buffer salt) and a microencapsulated plant-based phosphorus-nitrogen fire-retardant. Through the above scheme, the application effectively inhibits red change, retains the original color and texture of the wood, has excellent fire-retardant, weather-resistant and adhesive properties, and is environmentally friendly.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant coating technology, specifically, it relates to a plant-based transparent flame retardant coating for the protection of ancient buildings, its application, and preparation method. Background Technology

[0002] Given the unique value of historical and cultural heritage and the civilizational memories it carries, the fire safety and protection of timber-framed buildings have received unprecedented attention globally. As a natural organic material, wood exhibits excellent mechanical properties and aesthetic qualities in building structures; however, its inherent flammability makes it particularly vulnerable to fire risks. Traditional fire-fighting measures, such as sprinkler systems and fire barriers, are often insufficient for the non-invasive protection requirements of heritage buildings and may cause irreversible damage to their original appearance. Against this backdrop, fire-retardant coatings, especially transparent flame-retardant coatings, have become an important research direction and application solution in the field of timber-framed building heritage protection due to their ability to provide effective fire barriers while preserving the original texture and color of the wood to the greatest extent possible.

[0003] For a long time, the technological evolution in the field of flame-retardant coatings has revolved around improving flame-retardant efficiency, enhancing coating performance, and expanding application range. Early flame-retardant coatings were mostly non-intumescent or semi-transparent, their flame-retardant mechanism primarily involving the release of non-combustible gases or the formation of a dense glassy layer to isolate oxygen and heat. With continuous technological advancements, intumescent flame-retardant coatings have gradually become mainstream. These coatings undergo complex chemical and physical reactions upon heating, rapidly expanding to form a porous char layer, effectively delaying flame spread and reducing substrate temperature. Specifically, these coatings typically contain an acid source (such as ammonium polyphosphate), a carbon source (such as pentaerythritol or starch), and a gas source (such as melamine). These three components work synergistically to form a stable char structure at high temperatures. To meet aesthetic demands, especially for preserving the original appearance of wooden structures, transparent flame-retardant coatings have emerged. These coatings, while maintaining the original flame-retardant principles, achieve high transparency through precise control of the resin system, flame-retardant particle size, and dispersion. For example, by introducing modified melamine resin to improve water resistance and flexibility, using polymer resin as film-forming substrate, and carefully selecting phosphorus and nitrogen flame retardants to balance flame retardancy efficiency and transparency, and supplementing with a variety of functional additives such as waterproofing agents, antioxidants, weathering agents, adhesion promoters, and plasticizers, the overall performance of the coating is comprehensively improved, including film-forming effect, adhesion, transparency, weather resistance, water resistance, and durability. In particular, significant progress has been made in fire resistance and the density and adhesion of the char layer.

[0004] However, with the deepening of timber structure heritage preservation practices and the increasing emphasis on the "restoring the old as it was" concept, some inherent characteristics of existing transparent flame-retardant coatings at the principle level have gradually revealed their deep limitations in addressing specific challenges. Specifically, when traditional transparent flame-retardant coatings are applied to old timber, especially timber structural components that have been exposed to the natural environment for a long time, have high moisture content, and have undergone complex chemical aging, they often encounter unexpected aesthetic compatibility problems. This is not a simple coating defect, but a "secondary problem" caused by the subtle and complex interfacial reaction between the coating's chemical components and the old timber substrate. The reason for this is that the chemical composition of old timber differs significantly from that of newly felled timber. Under long-term weathering, sunlight, microbial erosion, and temperature and humidity cycles, the main components in old timber, such as cellulose, hemicellulose, and lignin, will degrade, producing more low-molecular-weight degradation products and active functional groups. Its pH value may also deviate from neutral. At the same time, various pigments, tannins, or metal ions accumulated inside the wood will also migrate and transform. When existing transparent flame-retardant coating systems, which may contain amine compounds, weakly acidic or weakly alkaline additives, and components that generate active intermediates during curing, come into contact with the chemically altered surfaces of old wood, they easily trigger redox reactions, complexation reactions, or acid-base catalytic reactions. Furthermore, in environments where old wood generally has a high moisture content, moisture, as an excellent solvent and reaction medium, significantly promotes these interfacial chemical reactions. As a result, after the coating cures, the surface of the old wood, especially the phenolic substances or lignin degradation products it contains, is prone to oxidative polymerization or reaction with other dye precursors under the synergistic effect of the coating system, forming dark or reddish-brown substances. This causes the wood to appear "reddish" beneath the originally transparent coating. This reddish discoloration not only destroys the original rustic color and texture of the wood but also contradicts the principle of "restoring the old as it was" pursued in cultural relic preservation, seriously affecting the aesthetic effect and historical authenticity of the restored product. This deep-seated chemical incompatibility constitutes an inherent bottleneck that existing technologies cannot avoid when dealing with specific application scenarios. Summary of the Invention

[0005] To address the technical challenge of existing transparent flame-retardant coatings used in the preservation of timber-framed architectural heritage, which cause a "red discoloration" phenomenon on the wood surface due to complex interfacial chemical reactions between the coating components and wood degradation products when applied to old or damp timber, thus deviating from the principle of "restoring the old as it was," this invention provides a plant-based transparent flame-retardant coating for the preservation of timber-framed architectural heritage. This coating system not only possesses excellent flame-retardant properties, weather resistance, and adhesion, but also effectively inhibits and eliminates the red discoloration phenomenon that may occur when applied to old or damp timber by introducing specific plant-based interfacial stabilizing components and reaction inhibition strategies. This ensures that the original color and texture of the wood are preserved to the maximum extent, realizing the authenticity and aesthetic value of the historical buildings.

[0006] This invention provides a plant-based transparent flame-retardant coating for the preservation of timber-framed architectural heritage. Its core lies in constructing a multifunctional, multi-layered composite protection system. This system comprises three main functional layers: a plant-based wood interface-stabilizing primer, a plant-based transparent intumescent flame-retardant intermediate coat, and a plant-based weather-resistant protective topcoat, supplemented by a precisely controlled wood pretreatment process. The various functional layers work synergistically to effectively retard old timber and inhibit red staining.

[0007] In a preferred embodiment of the present invention, the plant-based transparent flame-retardant coating is composed of the following components in weight percentage: (1) Plant-based film-forming resin: 25% to 45%; (2) Plant-based phosphorus and nitrogen flame retardants: 15% to 30%; (3) Plant-based carbon sources: 5% to 15%; (4) Plant-based interface stabilizers: 3% to 10%; (5) Plant-based antioxidants and weather-resistant agents: 2% to 8%; (6) Plant-based adhesion promoter: 1% to 5%; (7) Plant-based waterproofing agent: 1% to 5%; (8) Plant-based plasticizers: 1% to 5%; (9) Plant-based preservative and antifungal agent: 0.5% to 3%; (10) Plant-based defoaming stabilizer: 0.5% to 2%; (11) Plant-based crosslinking accelerator: 0.5% to 2%; (12) Deionized water: balance.

[0008] Furthermore, the plant-based film-forming resin is a composite resin copolymerized from cashew nut shell modified epoxy acrylate resin and modified cellulose ether. The preparation process of the cashew nut shell modified epoxy acrylate resin includes: etherifying cashew nut shell oil extracted from cashew nut shells with epichlorohydrin under alkaline conditions, followed by ring-opening esterification of the resulting cashew nut shell glycidyl ether with acrylic acid to obtain the cashew nut shell modified epoxy acrylate. This resin exhibits good water resistance, flexibility, and wood permeability. The modified cellulose ether is prepared by reacting plant cellulose with hydroxyethyl and hydroxypropyl etherifying agents under alkaline catalysis to obtain hydroxyethyl hydroxypropyl cellulose ether with a specific degree of substitution. By adjusting its degree of substitution, this cellulose ether ensures excellent transparency, film-forming properties, and compatibility with the cashew nut shell modified epoxy acrylate resin, together constituting the main film-forming substrate of this invention, providing the coating with basic mechanical strength and transparency. After curing, the composite resin can form a coating film with high transparency and good weather resistance. Furthermore, the phenolic hydroxyl and epoxy groups in its molecular structure are precisely controlled to reduce side reactions with active substances in old wood.

[0009] Furthermore, the plant-based phosphorus-nitrogen flame retardant is a microencapsulated composite intumescent flame retardant. Its preparation process includes three main steps: The first step involves the synthesis and modification of the phosphorus source. Phytic acid extracted from rice bran or corn is esterified and polymerized with phosphoric acid under high temperature conditions to form polymeric phytate phosphate (PPAE). This PPAE serves as the main phosphorus source and can decompose at high temperatures to produce phosphoric acid, promoting carbonization and dehydration of the wood surface.

[0010] The second step involves the synthesis and modification of the nitrogen source. Plant protein hydrolysates extracted from soybeans or corn are condensed with urea or guanidine salts under specific temperature and pressure conditions to generate a novel bio-based polyamide or polyguanidine salt derivative, which serves as the primary nitrogen source. This nitrogen source releases non-flammable gases (such as ammonia or nitrogen) upon heating, diluting the concentration of flammable gases and synergistically promoting carbonization with a phosphorus source.

[0011] The third step is microencapsulation. The synthesized PPAE and bio-based polyamide / guanidine salt derivatives are mixed in a specific ratio and then microencapsulated using interfacial polymerization or spray drying. The microcapsule wall material is selected from biodegradable polylactic acid (PLA) or polyhydroxybutyrate (PHB), with an average particle size controlled between 50 nm and 300 nm. Microencapsulation aims to isolate the flame retardant from direct contact with the wood, preventing unintended chemical reactions between the acidic or basic groups in the flame retardant and the active functional groups of old wood, thereby effectively inhibiting reddening and ensuring that the flame retardant can rapidly rupture and release the active ingredients when heated to high temperatures. The microcapsule wall material is selected from biopolymers with specific glass transition temperatures to ensure stability under normal conditions and rapid decomposition in the early stages of a fire.

[0012] Furthermore, the plant-based carbon source consists of modified starch and a pentaerythritol derivative. The modified starch is prepared by esterifying potato starch with citric acid or itaconic acid under specific conditions to obtain an acidic starch ester with high carbonization efficiency. This starch ester can be rapidly dehydrated and carbonized at high temperatures, providing a rich carbon skeleton for the expandable carbonized layer. The pentaerythritol derivative is prepared by esterifying fatty acids from vegetable oils (such as linseed oil and castor oil) with pentaerythritol to obtain a pentaerythritol fatty acid ester with multiple hydroxyl groups. This derivative not only serves as a carbon source but also as a plasticizer, improving the flexibility of the coating film. The carbon source works synergistically with phosphorus and nitrogen-based flame retardants to ensure the formation of a dense, high-strength, and excellent thermal insulation carbonized layer when heated.

[0013] Furthermore, the plant-based interface stabilizer is a key component of this invention for inhibiting the reddening of wood. It is composed of two main components working synergistically: a natural phenolic free radical scavenger and a plant-based chelating buffer salt.

[0014] The natural phenolic free radical scavenger is a polyphenolic compound rich in proanthocyanidins, catechins, and gallic acid, extracted from grape seeds or green tea using supercritical fluid extraction technology. This compound possesses excellent antioxidant capabilities, effectively capturing and quenching reactive free radicals generated on the surface of old wood under the action of coatings or under ultraviolet light and heat, thereby interrupting the oxidative polymerization reaction chain and inhibiting the formation of reddening substances. Its dispersion in the coating system is achieved through nanoemulsification technology, ensuring its uniform distribution and effective penetration into the shallow surface layer of the wood.

[0015] The plant-based chelated buffer salt is formed by neutralizing citric acid or oxalic acid with specific plant alkaloids (such as alkaline polypeptides extracted from quinoa) to create a complex with both pH buffering and metal ion chelating capabilities. During the coating curing process, this complex stabilizes the pH at the wood-coating interface, maintaining it within a near-neutral range and preventing the oxidation of phenolic substances in the wood due to acid-base fluctuations. Simultaneously, its chelating function immobilizes soluble transition metal ions such as iron and copper that may be present in aged wood, preventing these metal ions from acting as catalysts to accelerate the oxidative polymerization of wood degradation products, thus effectively inhibiting reddening. The particle size of this chelated buffer salt is controlled at the nanoscale to ensure its transparency.

[0016] Furthermore, the plant-based antioxidant and weather-resistant agent is composed of modified tocopherol and hydroxyl-rich plant polysaccharide crosslinkers. The modified tocopherol is obtained by chemically grafting natural vitamin E (tocopherol) with specific hydrophilic groups (such as polyethylene glycol segments), thereby improving its dispersibility and stability in water-based coating systems. This allows it to penetrate more effectively into the coating, providing long-lasting UV and antioxidant protection and delaying the aging and degradation of both the coating itself and the wood substrate. The hydroxyl-rich plant polysaccharide crosslinkers are, for example, formed by crosslinking gum arabic or guar gum with glutaraldehyde or epichlorohydrin under mild conditions to create a network-structured biomacromolecule. This crosslinker can absorb UV energy, and its abundant hydroxyl groups can form hydrogen bonds with the coating substrate, improving the density and hydrolysis resistance of the coating structure.

[0017] Furthermore, the plant-based adhesion promoter is a modified terpene resin. This terpene resin is prepared by reacting terpenoid compounds extracted from pine trees with maleic anhydride via a Diels-Alder reaction, followed by esterification with polyethylene glycol monomethyl ether. Its molecular structure contains polar maleic anhydride groups, capable of forming hydrogen bonds or covalent bonds with hydroxyl groups on the wood surface, and non-polar terpene segments, which are highly compatible with film-forming resin systems, thereby constructing a strong "molecular bridge" between the organic coating and the wood substrate, significantly improving the coating's adhesion. The molecular weight of this adhesion promoter is precisely controlled to ensure that it can effectively penetrate the wood surface while forming a stable colloid with the coating system.

[0018] Furthermore, the plant-based waterproofing agent is a modified silane emulsion. Its preparation process includes: surface esterification modification of nano-silica extracted from rice husk ash with salicylic acid to prepare hydrophobic nano-silica particles; subsequently, vinyltrimethoxysilane and octyltriethoxysilane are mixed at a specific molar ratio and subjected to partial hydrolysis and condensation reactions under dilute acid catalysis to form an oligomeric silane prepolymer; finally, the hydrophobic nano-silica particles and the oligomeric silane prepolymer are emulsified under high-speed shear, and a plant oil-based emulsifier and stabilizer are added to form a nano-scale silane emulsion with excellent stability and permeability. This waterproofing agent can form a hydrophobic protective layer on the wood surface, effectively preventing external moisture intrusion. Simultaneously, its nanostructure does not affect the transparency of the coating and can reduce the contact between the coating and residual moisture inside the wood, lowering the probability of water-assisted red staining reactions.

[0019] Furthermore, the plant-based plasticizer is triethyl citrate. This triethyl citrate is obtained by esterification of citric acid with ethanol in the presence of an acid catalyst, followed by purification through distillation. As an environmentally friendly plasticizer, it effectively lowers the glass transition temperature of the film-forming resin, improves the flexibility, crack resistance, and impact resistance of the coating, and also contributes to the smoothness and leveling of the coating, ensuring the formation of a uniform, dense, and transparent coating.

[0020] Furthermore, the plant-based preservative and anti-mildew agent is a complex essential oil microemulsion extracted from plants such as thyme and cinnamon. This microemulsion is formed by high-speed emulsification of the aforementioned plant essential oils with plant-derived surfactants (such as lecithin), resulting in a stable microemulsion with a particle size of less than 50 nanometers. The phenolic and aldehyde compounds it contains possess broad-spectrum antibacterial and antifungal activity, effectively inhibiting mold and decay fungi that easily grow on wood in humid environments, and preventing biodegradation damage to the coating and the wood.

[0021] Furthermore, the plant-based defoaming stabilizer is a modified castor oil-based polysiloxane. This defoamer is prepared by esterifying castor oil with an ethylene oxide / propylene oxide block copolymer, followed by graft copolymerization with polymethylsiloxane. It exhibits good surface activity and spreading ability, rapidly disintegrating bubbles generated during coating preparation and application, ensuring the smoothness and transparency of the coating film, while improving the storage stability of the coating system.

[0022] Furthermore, the plant-based crosslinking accelerator is an environmentally friendly polyamine derivative, such as a bio-polyamine (e.g., spermine, spermidine) extracted from seaweed, which is modified to undergo a rapid crosslinking reaction with specific functional groups (e.g., epoxy groups, carboxyl groups) in the film-forming resin system of the present invention under room temperature or micro-heat conditions, thereby accelerating the curing of the coating film and improving the hardness, wear resistance and chemical resistance of the coating film.

[0023] Furthermore, the deionized water is prepared using reverse osmosis membrane separation technology and deep treatment process with anion and cation exchange resins to ensure that its conductivity is less than 0.055 μS / cm and that it contains no soluble ions, organic matter, or microorganisms. This provides a pure solvent system for the preparation of the coating and avoids the introduction of impurities that could cause side effects.

[0024] The method for preparing the plant-based transparent flame-retardant coating provided by this invention includes the following steps: (1) Synthesis and microencapsulation of plant-based phosphorus and nitrogen flame retardants: a. Phosphorus source synthesis: 90 to 110 parts of phytic acid and 50 to 70 parts of phosphoric acid are added to a reactor equipped with a stirrer, condenser, and heater. Under a nitrogen atmosphere, the mixture is heated to 150 to 170°C at a rate of 2 to 3°C per minute, and the reaction is continued at this temperature for 3 to 5 hours to form polymeric phytic acid phosphate (PPAE). Temperature is strictly controlled during the reaction to avoid side reactions.

[0025] b. Nitrogen source synthesis: 80 to 100 parts of plant protein hydrolysate, 40 to 60 parts of urea, and 2 to 5 parts of zinc acetate as a catalyst are added to another reactor. Under stirring, the mixture is heated to 130 to 140°C at a heating rate of 1.5 to 2.5°C per minute and held at this temperature for 4 to 6 hours to generate a bio-based polyamide derivative. Ammonia gas generated during the reaction is recovered and treated using a gas absorption device.

[0026] c. Microencapsulation: The PPAE obtained in step (1)a and the bio-based polyamide derivative obtained in step (1)b are mixed at a mass ratio of 1:1 to 1.5:1 and added to a high-speed dispersion vessel. The mixture is dispersed at 8000 rpm to 12000 rpm for 30 minutes to form a uniform dispersion. Subsequently, 150 to 200 parts of polylactic acid (PLA) are dissolved in 1000 to 1500 parts of dichloromethane to form a capsule wall material solution. Under vigorous stirring, the flame retardant dispersion is slowly added dropwise to the PLA solution, and microencapsulation is performed by interfacial polymerization or water-in-oil emulsion method. The reaction temperature is controlled at 25°C to 35°C, and stirring is continued for 6 to 8 hours. Subsequently, through solvent evaporation, filtration, washing, and vacuum drying steps, microencapsulated plant-based phosphorus-nitrogen flame retardant powder with an average particle size in the range of 50 nm to 300 nm is obtained.

[0027] (2) Synthesis of plant-based film-forming resins: a. Preparation of cashew nut shell phenol-modified epoxy acrylate resin: In a four-necked flask equipped with a stirrer, condenser, dropping funnel, and thermometer, 100 parts of cashew nut shell phenol and 50 to 70 parts of sodium hydroxide aqueous solution were added. The mixture was heated to 60°C, and 90 to 110 parts of epichlorohydrin were slowly added dropwise over 2 hours, maintaining the reaction temperature at 80 to 90°C for 4 hours. The mixture was cooled to 50°C, filtered to remove salt, and washed with water until neutral to obtain cashew nut shell phenol glycidyl ether. Subsequently, the cashew nut shell phenol glycidyl ether was reacted with 40 to 50 parts of acrylic acid in the presence of 0.1 parts of triphenylphosphine catalyst and 2 to 3 parts of p-methoxyphenol polymerization inhibitor, and heated to 100 to 120°C for 5 to 7 hours until the acid value reached the target range, to obtain cashew nut shell phenol-modified epoxy acrylate resin.

[0028] b. Preparation of modified cellulose ether: 80 to 120 parts of plant cellulose pulp were mixed with 1000 to 1500 parts of isopropanol and 50 to 70 parts of sodium hydroxide aqueous solution, and alkalized at 40°C for 1 hour. Subsequently, a mixture of 40 to 60 parts of ethylene oxide and 30 to 50 parts of propylene oxide was added dropwise, and the mixture was reacted at 70 to 80°C for 3 to 5 hours. After the reaction was completed, the mixture was neutralized to pH 6-7 with dilute acetic acid, filtered, washed, and dried to obtain modified hydroxyethyl hydroxypropyl cellulose ether.

[0029] c. Preparation of composite resin: The cashew phenol modified epoxy acrylate resin obtained in step (2)a and the modified cellulose ether obtained in step (2)b are mixed at a mass ratio of 1.5:1 to 2.5:1 and stirred at 50°C to 60°C for 2 hours to form a uniform plant-based composite resin.

[0030] (3) Preparation of plant-based interface stabilizers: a. Extraction and Nanoemulsification of Natural Phenolic Free Radical Scavengers: 100 parts grape pomace were mixed with 1000 to 1500 parts of a 50% ethanol aqueous solution. The mixture was extracted for 6 hours using a supercritical carbon dioxide extraction device at a pressure of 35 to 45 MPa and a temperature of 40 to 50°C. The extract was collected and concentrated to obtain a crude extract rich in proanthocyanidins. The crude extract was dissolved in ethanol and mixed with plant-derived emulsifiers such as lecithin and polysorbate 80 in a specific ratio. The mixture was then homogenized five times using a high-pressure homogenizer at a pressure of 100 to 150 MPa to prepare a natural phenolic free radical scavenger nanoemulsion with a particle size of less than 50 nanometers.

[0031] b. Synthesis of plant-based chelated buffer salts: 80 to 120 parts of citric acid were neutralized with an alkaline polypeptide solution (polypeptide content 20% to 30%) extracted from quinoa in an equimolar ratio. The reaction was carried out in an aqueous solution at a temperature controlled at 25°C to 35°C and stirred for 1 hour. Subsequently, plant-based citric acid-polypeptide chelated buffer salt powder with a particle size of less than 100 nanometers was obtained by spray drying or freeze drying.

[0032] (4) Overall preparation of plant-based transparent flame-retardant coatings: a. Dispersion stage: Add 70% to 80% of the total deionized water to a high-speed dispersion vessel. While stirring, add the plant-based film-forming resin, plant-based interface stabilizer (nanoemulsion and buffer salt powder), plant-based antioxidant and weather-resistant agent, plant-based adhesion promoter, and plant-based plasticizer / film-forming agent in sequence. Dispersion speed is 1500 rpm to 2500 rpm, and dispersion time is 30 to 60 minutes to ensure that all components are fully wetted and uniformly dispersed.

[0033] b. Grinding Stage: Transfer the above dispersion system to a sand mill, add microencapsulated plant-based phosphorus-nitrogen flame retardant and plant-based carbon source, and use 0.8mm to 1.2mm zirconia beads as the grinding media. Under cooling circulation conditions, grind at a linear speed of 2500rpm to 3500rpm for 2 to 4 hours to ensure that the solid particle size reaches less than 500nm, thus guaranteeing the transparency of the coating. Strictly control the temperature during grinding, not exceeding 40℃.

[0034] c. Mixing stage: Transfer the ground slurry to a mixing tank, add the remaining deionized water, plant-based waterproofing agent, plant-based preservative and mildew-preventing agent, plant-based defoaming stabilizer, and plant-based crosslinking accelerator. Stir at 500 rpm to 800 rpm for 30 to 60 minutes to ensure all components are thoroughly mixed.

[0035] d. Curing and Filtration: The prepared coating is allowed to stand at 25°C for 12 to 24 hours to cure, ensuring that all components react fully and stabilize. Subsequently, it is filtered through a 100-200 mesh filter to remove any possible minute impurities or incompletely dispersed particles, obtaining the final plant-based transparent flame-retardant coating product.

[0036] The application method of the plant-based transparent flame-retardant coating provided by this invention is mainly aimed at the protection of timber-framed architectural heritage. Its process includes: (1) Wood Substrate Testing and Evaluation: Before coating application, a comprehensive and detailed testing and evaluation of the old wood substrate is conducted. A non-contact infrared thermometer and a portable wood moisture meter are used to accurately measure the surface temperature and internal moisture content of the wood. An ultrasonic flaw detector is used to check for internal voids, decay, or termite infestation. The wood surface texture, degree of damage, and presence of weathering, cracking, discoloration, etc., are visually inspected. Based on the test results, a personalized pretreatment and construction plan is developed for each wooden component.

[0037] (2) Wood surface cleaning and "open-mouth drainage method": a. Preliminary cleaning: Use a soft-bristled brush and a low-pressure air gun to remove dust, loose material, and other debris from the wood surface. For stubborn stains, use a neutral pH plant-based cleaner with a soft cloth or sponge to gently wipe the surface. Avoid using strong acid or alkali cleaners that could damage the wood.

[0038] b. "Live Drainage Method": For damp, old wood with a moisture content exceeding 18%, this invention employs a unique "live drainage method" for drying. This method involves selectively drilling micro-holes, 5 mm to 8 mm in diameter and two-thirds to three-quarters of the wood's thickness, in concealed areas of the wooden components (such as mortise and tenon joints or the shaded side). The spacing between these holes is set at 20 cm to 30 cm. These holes act as "live openings," accelerating the diffusion of moisture from the wood's interior through natural convection and capillary action. To further improve drainage efficiency, a small amount of highly absorbent plant fiber material (such as corn starch or cotton cellulose) or inert desiccant granules can be filled inside the holes. Simultaneously, a micro-ventilation device (such as a low-power, low-speed axial flow fan) is installed on the wood surface to create gentle airflow, promoting moisture evaporation. The wood's moisture content is continuously monitored until it drops to the ideal range of 8% to 15%. This method effectively avoids the stress damage and cracking risks caused by traditional drying methods, maximizing the protection of the wood's original structure.

[0039] (3) Application of plant-based wood interface stabilizing primer: a. Dilute the above-prepared plant-based transparent flame-retardant coating, wherein the concentration of the plant-based interface stabilizer is increased to 15% to 20% (based on solid content), and use it as a primer.

[0040] b. Apply the plant-based wood interface stabilizing primer evenly to the cleaned and dried wood surface using a brush, roller, or low-pressure spray method. The coating thickness should be controlled between 30 and 50 micrometers.

[0041] c. After coating, allow to surface dry for 30 to 60 minutes and fully dry for 4 to 6 hours under ambient temperature of 20°C to 25°C and relative humidity of 50% to 70%. The natural phenolic free radical scavenger in this primer can penetrate to the surface of the wood, effectively passivating active phenolic substances and degradation products in the wood, chelating any metal ions that may be present, and stabilizing the pH value at the interface, thereby inhibiting the occurrence of red staining reaction from the source.

[0042] (4) Application of plant-based transparent intumescent flame-retardant intermediate coating: a. After the primer has completely dried, apply multiple layers of undiluted plant-based transparent flame retardant coating as the intermediate coat using a brush or roller.

[0043] b. Each coating layer should be 80 to 120 micrometers thick, with an interval of 6 to 8 hours between layers. Ensure the previous layer is completely dry before applying the next. The total number of coating layers depends on the required flame retardancy rating and wood type, typically 2 to 3 layers. The total thickness should be 200 to 300 micrometers.

[0044] c. The microencapsulated plant-based phosphorus and nitrogen flame retardants and plant-based carbon sources in the intermediate coating are evenly distributed and can rapidly expand to form a dense and stable char layer when a fire occurs, effectively isolating heat and oxygen and delaying the spread of flames.

[0045] (5) Application of plant-based weather-resistant protective topcoat: a. After the intermediate coat has fully cured (usually requiring 24 to 48 hours), the plant-based transparent flame-retardant coating of the present invention is diluted again, and the ratio of plant-based antioxidant and weather-resistant agent and plant-based waterproofing agent is adjusted to increase its solid content in the topcoat to 10% to 15% as a protective topcoat.

[0046] b. Apply the topcoat evenly to the surface of the flame-retardant intermediate coating using low-pressure spraying or brushing, with the coating thickness controlled between 40 and 60 micrometers.

[0047] c. After the topcoat is applied, cure for 24 to 48 hours at an ambient temperature of 20°C to 25°C and a relative humidity of 50% to 70%. This topcoat forms a tough, wear-resistant, highly transparent protective layer with excellent weather resistance and water resistance, effectively resisting ultraviolet rays, rain, humidity changes, and microbial erosion, extending the service life of the entire coating system, while maintaining the long-term aesthetic effect of the wood.

[0048] The technical principle of the plant-based transparent flame-retardant coating for the protection of timber-framed architectural heritage provided by this invention is as follows: This coating system achieves flame retardancy and red stain inhibition through multifunctional synergy. Firstly, during the wood pretreatment stage, a precisely controlled "open-mouth drainage method" effectively reduces the moisture content of old wood, minimizing the possibility of moisture acting as a reaction medium and triggering interfacial reactions. Secondly, the plant-based wood interface stabilizing primer is key to inhibiting red stain. Its contained natural phenolic free radical scavengers can capture any free radicals present in the wood, interrupting oxidative polymerization reactions caused by light, heat, or the active catalysis of coating components. Simultaneously, the plant-based chelating buffer salt plays a dual role: on the one hand, its buffering capacity stabilizes the pH value at the wood-coating interface within a near-neutral range, preventing oxidative staining of phenolic substances in the wood due to fluctuations in acidity and alkalinity; on the other hand, its chelating function effectively complexes transition metal ions (such as iron and copper) migrating from old wood, preventing these metal ions from acting as catalysts for redox reactions, thereby inhibiting the formation of red staining substances at the molecular level. Furthermore, the carefully selected plant-based film-forming resin in the primer undergoes precise modification, optimizing its chemical activity and reducing unintended reactions with degradation products of old wood.

[0049] In the intermediate coating layer, microencapsulated plant-based phosphorus and nitrogen flame retardants play a key role in flame retardancy. This flame retardant isolates the active phosphorus and nitrogen components from the wood substrate through its microencapsulation structure, preventing direct contact between the acidic or alkaline components of the flame retardant and the old wood, further reducing the risk of red discoloration caused by interfacial reactions. Under high-temperature fire conditions, the microcapsule walls rapidly rupture, releasing phosphate and nitrogen sources. The phosphate source catalyzes the dehydration and carbonization of the wood and plant-based carbon sources at high temperatures, forming a dense carbonized layer. This carbonized layer has excellent heat insulation and oxygen barrier properties, effectively preventing the flame from spreading into the substrate. The nitrogen source decomposes and releases non-combustible gases, diluting the concentration of combustible gases in the flame area and further inhibiting combustion. The plant-based carbon source and the phosphorus and nitrogen flame retardant work synergistically to provide abundant carbonization precursors, ensuring the formation of a carbonized layer with high expansion ratio, stable structure, and strong adhesion, effectively protecting the wood substrate.

[0050] The topcoat layer provides comprehensive weather resistance and waterproof protection. Modified tocopherols and plant polysaccharide crosslinks in the plant-based antioxidant weathering agent effectively absorb ultraviolet light, resist oxidative degradation, and delay the aging of both the coating itself and the wood substrate. The nano-silane emulsion in the plant-based waterproofing agent forms a hydrophobic layer on the surface, effectively preventing external moisture erosion and further reducing fluctuations in wood moisture content, thereby indirectly reducing the possibility of red staining. Plant-based adhesion promoters ensure a strong bond between each coating layer and the wood, as well as between coating layers, guaranteeing the long-term stability and integrity of the coating system.

[0051] The plant-based transparent flame-retardant coating and its preparation and application methods provided by this invention have the following beneficial effects: First, it significantly inhibits the reddening phenomenon of old wood: By introducing plant-based interface stabilizers (natural phenolic free radical scavengers and plant-based chelating buffer salts) and microencapsulated flame retardants, this invention can effectively passivate the active groups of wood at the molecular level, stabilize the interface pH, chelate metal ions, and isolate active components, thereby fundamentally inhibiting and eliminating the reddening phenomenon that may occur when coating old, damp wood, preserving the original color and texture of the wood to the greatest extent, which is in line with the principle of "restoring the old as it was" in cultural relic protection.

[0052] Secondly, excellent flame retardant performance: This invention uses a highly efficient microencapsulated plant-based phosphorus and nitrogen-based intumescent flame retardant that works synergistically with a plant-based carbon source. In the event of a fire, it can quickly form a carbonized layer with a high expansion ratio, dense structure, high strength, and strong adhesion, effectively isolating heat and oxygen, significantly improving the fire resistance limit and flame retardant rating of wood, and providing reliable fire safety protection for wooden architectural heritage.

[0053] Third, excellent overall performance: The coating system of this invention has excellent transparency and does not affect the natural beauty of the wood after application. At the same time, by introducing a variety of functional additives such as plant-based antioxidants, plant-based waterproofing agents, and plant-based adhesion promoters, the weather resistance, water resistance, adhesion, flexibility, hardness, and durability of the coating film are significantly improved, ensuring that the coating can stably protect the wood for a long time.

[0054] Fourth, environmental friendliness and sustainability: This invention extensively uses plant-based raw materials, including plant-based film-forming resins, plant-based phosphorus and nitrogen flame retardants, plant-based carbon sources, plant-based interface stabilizers, plant-based antioxidants and weather-resistant agents, plant-based adhesion promoters, plant-based waterproofing agents, plant-based plasticizers and film-forming agents, plant-based preservatives and mildew inhibitors, plant-based defoaming stabilizers, and plant-based crosslinking promoters, etc., which reduces dependence on petrochemical products and reduces emissions of volatile organic compounds (VOCs), in line with the concept of green environmental protection and sustainable development, and is especially suitable for the field of cultural relic protection with strict environmental requirements.

[0055] Fifth, precise construction application solutions: This invention provides a "live-hole drainage method" and a multi-layer coating strategy for old wood, which can be customized according to the specific condition of the wood to ensure the maximum performance of the coating, while avoiding secondary damage to the cultural relics wood, thus improving the adaptability and reliability of construction.

[0056] Sixth, synergistic effect of the system: This invention is not a simple superposition of single components, but rather a precise design and synergistic effect of a three-layer coating system (interface-stabilizing primer, flame-retardant intermediate coat, and weather-resistant protective topcoat). Each layer complements the function of the others, working together to address the complex challenges of old wood and achieving an overall functional improvement. The primer is responsible for interface stabilization to inhibit red discoloration, the intermediate coat provides the core flame-retardant function, and the topcoat provides long-lasting weather resistance and waterproof protection, ensuring the comprehensiveness and long-term effectiveness of wood heritage protection. Detailed Implementation

[0057] This invention provides a plant-based transparent flame-retardant coating for the preservation of timber-framed architectural heritage. Its design aims to effectively address the "red discoloration" phenomenon commonly observed with traditional coatings applied to old, damp wood, while simultaneously endowing the wood with superior flame-retardant, weather-resistant, and aesthetic protection. This coating system is not a simple superposition of single components, but rather, through ingenious structural design and component synergy, constructs a composite protective layer comprising a plant-based wood interface-stabilizing primer, a plant-based transparent intumescent flame-retardant intermediate coat, and a plant-based weather-resistant protective topcoat. This is further enhanced by precisely controlled wood pretreatment processes, ensuring that the original color and texture of the wood are preserved to the greatest extent possible, thus achieving a balance between the authenticity and aesthetic value of the cultural relic.

[0058] In a preferred embodiment of the present invention, the constituent components and their mass percentages of the plant-based transparent flame-retardant coating are as follows: plant-based film-forming resin accounts for 25% to 45%; plant-based phosphorus and nitrogen-based flame retardant accounts for 15% to 30%; plant-based carbon source content is 5% to 15%; plant-based interface stabilizer accounts for 3% to 10%; plant-based antioxidant and weather-resistant agent is added at 2% to 8%; plant-based adhesion promoter accounts for 1% to 5%; plant-based waterproofing agent accounts for 1% to 5%; plant-based plasticizer and film-forming agent accounts for 1% to 5%; plant-based preservative and mildew-resistant additive accounts for 0.5% to 3%; plant-based defoaming stabilizer accounts for 0.5% to 2%; plant-based crosslinking promoter accounts for 0.5% to 2%; and the balance is made up by deionized water. The precise proportions and synergistic effects between the components are key to achieving the performance indicators described in this invention.

[0059] Specifically, the plant-based film-forming resin is a composite resin copolymerized from cashew nut shell modified epoxy acrylate resin and modified cellulose ether. The preparation process of its main component, cashew nut shell modified epoxy acrylate resin, strictly follows a multi-step chemical reaction and purification process. First, cashew nut shell oil, selected from cashew nut shells, is etherified with epichlorohydrin under weakly alkaline conditions under an inert gas atmosphere. A typical procedure involves adding a predetermined amount of cashew nut shell oil to a reaction vessel, precisely controlling the temperature between 60°C and 80°C, and slowly adding a pre-mixed aqueous solution of epichlorohydrin and sodium hydroxide over a period typically of 2 to 3 hours. The reaction mixture is then continuously stirred at 80°C to 90°C for 4 to 6 hours to ensure complete etherification. After the reaction, the generated inorganic salts are removed by centrifugation or filtration. The resulting crude product is repeatedly washed with deionized water until the pH is neutral, and then vacuum dried to remove moisture and residual solvent, thereby obtaining high-purity cashew nut shell glycidyl ether. Subsequently, the obtained cashew phenol glycidyl ether was transferred to another reaction vessel. Under the conditions of adding 0.1 to 0.3 parts (based on the mass of cashew phenol glycidyl ether) of triphenylphosphine as a catalyst and 2 to 4 parts (based on the mass of cashew phenol glycidyl ether) of p-methoxyphenol as a polymerization inhibitor, a ring-opening esterification reaction was carried out with precisely measured acrylic acid. The reaction temperature was controlled at 100°C to 120°C, and the reaction was continued for 5 to 7 hours. The acid value of the reaction system was monitored until it reached the preset target range (usually below 5 mg KOH / g), indicating that the acrylic acid had fully reacted with the epoxy groups. After the reaction, the cashew phenol-modified epoxy acrylate resin was obtained through cooling and filtration. This resin, due to the hydrophobicity and flexibility imparted by the cashew phenol structure, and the good film-forming properties and water resistance provided by the epoxy acrylate skeleton, exhibits excellent penetration and adhesion to wood.

[0060] Furthermore, another key component of the composite resin is the modified cellulose ether. Its preparation typically involves the etherification modification of plant cellulose. High-purity plant cellulose pulp, such as wood pulp or cotton pulp, is pretreated in an inert solvent such as isopropanol or tert-butanol to disperse and activate the cellulose. Subsequently, an alkalization treatment is carried out by adding a precisely measured amount of aqueous sodium hydroxide solution under stirring, typically at 40°C to 50°C for 1 to 2 hours, to form alkali cellulose. Next, a pre-mixed mixture of hydroxyethyl etherifying agent (such as ethylene oxide) and hydroxypropyl etherifying agent (such as propylene oxide) is slowly added dropwise, and the etherification reaction is typically carried out at 70°C to 80°C for 3 to 5 hours. After the reaction, the system is neutralized to pH 6 to 7 by adding dilute acetic acid or other weak acid, followed by repeated washing to remove unreacted reagents and byproducts. Finally, hydroxyethyl hydroxypropyl cellulose ether with a specific degree of substitution is obtained by filtration and drying (such as vacuum drying or spray drying). The degree of substitution of this modified cellulose ether is precisely controlled to ensure excellent solubility, transparency, and film-forming properties in aqueous systems, and good compatibility with cashew phenol-modified epoxy acrylate resin. These two resins, through intermolecular interactions, together constitute the basic film-forming substrate of the coating system of this invention, forming a coating film with high transparency and good weather resistance after curing. Notably, the activity of the phenolic hydroxyl and epoxy groups in this composite resin has been precisely controlled in its molecular design to reduce potential unintended interfacial side reactions with degradation products in aged wood, thus laying the foundation for subsequent red stain inhibition.

[0061] Furthermore, the plant-based phosphorus-nitrogen flame retardant is a microencapsulated composite intumescent flame retardant. Its preparation process encompasses the synthesis of phosphorus and nitrogen sources followed by microencapsulation.

[0062] The first step is the synthesis and modification of the phosphorus source. Phytic acid extracted from rice bran or corn and industrial-grade phosphoric acid are added to a reactor equipped with a mechanical stirrer, reflux condenser, and temperature control device in a precise ratio, for example, the molar ratio of phytic acid to phosphoric acid is controlled between 1:1 and 1:1.5. Under a continuous nitrogen protective atmosphere, the system is slowly heated to 150°C to 170°C at a rate of 2°C to 3°C per minute. At this high temperature, the reaction mixture is continuously stirred for 3 to 5 hours. During this process, phytic acid and phosphoric acid undergo esterification and polymerization reactions to generate poly(PPAE). After the reaction is completed, insoluble matter is removed by cooling, centrifugation or filtration, followed by washing with water and drying to obtain a white or pale yellow powder of PPAE. As the main phosphorus source, PPAE contains abundant phosphate groups in its molecular structure. When heated at high temperatures, it can decompose to produce phosphoric acid or polyphosphoric acid. These acidic substances can catalyze the dehydration and carbonization of carbon sources in wood and coating systems, thereby promoting the formation of a carbon layer.

[0063] The second step involves the synthesis and modification of the nitrogen source. Plant protein hydrolysates extracted from soybeans or corn, with a degree of hydrolysis typically controlled between 60% and 80%, are added to another reactor along with urea or guanidine salts at a mass ratio of 1:0.8 to 1:1.2, and 2 to 5 parts (based on the mass of the plant protein hydrolysates) of zinc acetate as a catalyst. Under continuous stirring, the reaction system is heated to 130°C to 140°C at a rate of 1.5°C to 2.5°C per minute. At this temperature, the reaction is maintained for 4 to 6 hours, allowing the plant protein hydrolysates to undergo a complex condensation reaction with urea or guanidine salts, generating novel bio-based polyamides or polyguanidine salt derivatives. Ammonia or other volatile substances generated during the reaction are recovered using a specialized gas absorption device to ensure environmental friendliness and process safety. After the reaction, the nitrogen source component is obtained as a white or off-white powder through cooling, filtration, washing, and drying. When heated, this nitrogen source can release a large amount of non-combustible gases, such as ammonia and nitrogen. These gases can effectively dilute the concentration of combustible gases in the flame area, thereby playing a role in gas-phase flame retardancy and synergistically promoting the carbonization process with the phosphorus source.

[0064] The third step is microencapsulation. The synthesized PPAE phosphorus source and bio-based polyamide / polyguanidine salt derivative nitrogen source are precisely weighed at a mass ratio of 1:1 to 1.5:1 and added to a high-speed shear dispersion vessel. Dispersion is carried out continuously at 8000 rpm to 12000 rpm for 30 minutes to form a uniform and stable flame retardant composite dispersion. Simultaneously, 150 to 200 parts (based on the total mass of the phosphorus and nitrogen flame retardants) of biodegradable polylactic acid (PLA) or polyhydroxybutyrate (PHB) are dissolved as the capsule wall material in 1000 to 1500 parts of dichloromethane, ethyl acetate, or other suitable solvents to form a capsule wall material solution with a concentration of 10% to 15% (w / v). Subsequently, under vigorous stirring (e.g., by high-speed dispersion or high-pressure homogenization equipment), the previously prepared flame retardant composite dispersion is slowly added dropwise to the capsule wall material solution at a constant rate. The microencapsulation process can be achieved using interfacial polymerization or water-in-oil emulsion solvent evaporation. The reaction temperature must be strictly controlled between 25℃ and 35℃, and stirring must be continuous for 6 to 8 hours to ensure the formation and stability of microcapsules. After microencapsulation, residual solvent and unencapsulated components are removed by solvent evaporation, centrifugation or filtration, and repeated washing with deionized water or dilute ethanol. Finally, the powder is dried in a vacuum drying oven at 30℃ to 40℃ for 24 to 48 hours to obtain microencapsulated plant-based phosphorus-nitrogen flame retardant powder with an average particle size controlled between 50 nm and 300 nm. The core purpose of microencapsulation is to physically isolate the active components of the flame retardant from the wood substrate, thereby effectively preventing unintended chemical reactions between the acidic or basic groups in the flame retardant and the active functional groups in the degradation products on the surface of old wood, and significantly inhibiting the occurrence of red staining. Meanwhile, the capsule wall material of the microcapsule is a biopolymer with a specific glass transition temperature (Tg), which ensures that it has sufficient stability in normal temperature and humidity environments. When it encounters the high temperature conditions at the beginning of a fire, the capsule wall can quickly soften, melt or rupture, and release the active flame retardant components inside in time, thereby achieving high-efficiency flame retardancy.

[0065] Furthermore, the plant-based carbon source consists of modified starch and pentaerythritol derivatives. The modified starch is typically prepared by esterifying potato starch, corn starch, or other plant starches with organic acids such as citric acid or itaconic acid in a mass ratio of 100:1 to 100:5 under specific conditions. For example, starch is mixed with citric acid and heated at 120°C to 150°C for 2 to 4 hours under dry conditions, or reacted in an aqueous solution with a catalyst, followed by washing and drying to obtain an acidic starch ester with high carbonization efficiency. The carboxylic acid groups or their esters introduced into the starch ester molecule can rapidly dehydrate and carbonize at high temperatures, providing a rich carbon skeleton for the formation of an expanded carbonized layer. The pentaerythritol derivative is prepared by esterifying fatty acids extracted from vegetable oils (such as linseed oil and castor oil), such as linolenic acid and ricinoleic acid, with pentaerythritol in the presence of an acidic catalyst (such as p-toluenesulfonic acid) at 180°C to 220°C for 4 to 6 hours. During the reaction, the generated reaction water is removed via an azeotropic method until the acid value reaches a preset value, yielding a pentaerythritol fatty acid ester with multiple hydroxyl groups. This derivative not only serves as a highly efficient carbon source, but its polyhydroxy structure also reacts with phosphoric acid during carbonization to promote cross-linking and carbonization. Simultaneously, its fatty acid segments impart a certain degree of flexibility to the coating film, acting as a plasticizer to improve its mechanical properties. These plant-based carbon sources, in synergy with the aforementioned plant-based phosphorus-nitrogen flame retardants, ensure the rapid formation of a dense, high-strength, heat-insulating, and strongly adhesive intumescent carbonized layer when the coating is heated.

[0066] Furthermore, the plant-based interface stabilizer is the core functional component of this invention for effectively inhibiting the reddening of wood, achieved through the synergistic effect of two main components. First, there is the natural phenolic free radical scavenger, whose extraction and application processes are meticulously designed. This scavenger is extracted from plant materials such as grape seeds, green tea, or pine bark using supercritical fluid extraction technology, yielding polyphenolic compounds rich in proanthocyanidins, catechins, gallic acid, and other flavonoids. Typical extraction conditions include: extraction for 6 to 8 hours using supercritical carbon dioxide as the extraction solvent at a pressure of 35 MPa to 45 MPa and a temperature of 40°C to 50°C, followed by vacuum separation to obtain a high-purity crude polyphenol extract. To ensure its uniform dispersion in water-based coating systems and effective penetration into the shallow surface of wood, this crude extract requires further nano-emulsification treatment. Typically, the crude extract is dissolved in a small amount of environmentally friendly solvents such as ethanol or propylene glycol, and then mixed with plant-derived emulsifiers such as lecithin, polysorbate 80, and sucrose esters in a specific ratio. The mixture is then homogenized 5 to 10 times using a high-pressure homogenizer at a pressure of 100 to 150 MPa to prepare a stable nanoemulsion with a particle size of less than 50 nanometers. The polyphenolic compounds in this nanoemulsion possess excellent antioxidant capabilities, effectively capturing and quenching reactive oxygen and carbon free radicals that may be generated on the surface of old wood under environmental factors (such as ultraviolet radiation and heat radiation) or the influence of coating components. This effectively interrupts the oxidative polymerization reaction chain initiated by free radicals, inhibiting the formation of colored substances that cause wood to turn red at the molecular level.

[0067] Secondly, there are plant-based chelated buffer salts. Their preparation typically involves a precise neutralization reaction between plant-derived organic acids such as citric acid, oxalic acid, or tartaric acid and specific plant alkaloids (such as peptides rich in basic amino acids like lysine and arginine extracted from quinoa, soybeans, or yeast). For example, 80 to 120 parts of citric acid are dissolved in deionized water and mixed with 20% to 30% (w / v) of an aqueous peptide solution at an equimolar ratio or a slightly alkaline ratio (pH 7.0-7.5). The reaction is carried out under mild conditions of 25°C to 35°C with continuous stirring for 1 to 2 hours. The reaction product is then obtained by spray drying or freeze drying to obtain plant-based citric acid-peptide chelated buffer salt powder with a particle size controlled at the nanoscale (less than 100 nanometers). This complex can precisely stabilize the pH value at the wood-coating interface during the coating curing process, maintaining it within a near-neutral range, thereby effectively preventing non-enzymatic oxidative coloring reactions of phenolic substances inside the wood caused by drastic fluctuations in interface pH. Meanwhile, its molecular structure contains multiple carboxyl and amino groups, giving it a strong metal ion chelating ability. It can efficiently complex soluble transition metal ions such as iron, copper, and manganese that may be present in old wood. By immobilizing these metal ions, it can effectively prevent them from acting as catalysts to accelerate the oxidative polymerization reaction of wood degradation products, further enhancing the inhibitory effect on red staining. This nanoscale chelated buffer salt ensures its transparency in the coating and will not negatively affect the visual appearance of the wood.

[0068] Furthermore, the plant-based antioxidant and weather-resistant agent is composed of modified tocopherol and a cross-linked plant polysaccharide rich in hydroxyl groups. The modified tocopherol is prepared by chemically grafting natural vitamin E (tocopherol) with specific hydrophilic groups (such as polyethylene glycol monomethyl ether, polyoxyethylene sorbitan fatty acid esters, etc.), thereby endowing it with stronger hydrophilicity and significantly improving its dispersibility and stability in water-based coating systems. Typical modification reactions can be achieved under mild conditions through esterification or etherification reactions, for example, reacting tocopherol with polyethylene glycol dianhydride. The modified tocopherol can penetrate more effectively into the coating and distribute evenly, providing long-lasting UV absorption and free radical scavenging capabilities, thereby delaying the aging and degradation of the coating itself and the wood substrate caused by photo-oxidation and thermal oxidation. The hydroxyl-rich plant polysaccharide crosslinkers, such as those formed by reacting plant polysaccharides like gum arabic, guar gum, xanthan gum, or sodium carboxymethyl cellulose with glutaraldehyde, epichlorohydrin, or polycarboxylic acids (such as citric acid or butanetetracarboxylic acid) under mild alkaline or acidic conditions, create biomacromolecules with a network structure. The conditions of the crosslinking reaction (such as temperature, pH, and crosslinking agent concentration) need to be precisely controlled to obtain products with specific degrees of crosslinking and water-swelling properties. Due to the large number of hydroxyl groups in its molecular structure, this crosslinker can not only form a strong bond with the coating substrate through hydrogen bonds, improving the density and hydrolysis resistance of the coating structure, but also effectively absorb ultraviolet energy, converting it into harmless heat energy, thereby reducing UV damage to the coating and wood and improving the long-term weather resistance of the coating.

[0069] Furthermore, the plant-based adhesion promoter is a modified terpene resin. The preparation process of this terpene resin involves two main chemical transformations. First, terpenoid compounds such as α-pinene, β-pinene, or limonene extracted from pine trees are reacted with maleic anhydride at high temperatures (e.g., 180°C to 220°C) using a Diels-Alder reaction. This reaction is typically carried out under an inert atmosphere by heating under reflux for 4 to 6 hours, generating a terpene-maleic anhydride adduct with maleic anhydride groups. Subsequently, the resulting adduct is esterified with polyethylene glycol monomethyl ether (typically with a molecular weight of 200 to 600) at 120°C to 160°C in the presence of an esterification catalyst (e.g., p-toluenesulfonic acid). The water generated in the reaction is removed by an azeotropic method until the acid value drops to the target range (typically below 10 mg KOH / g), yielding the modified terpene resin. The resin's molecular structure cleverly incorporates polar maleic anhydride ester groups. These groups can form strong interactions with the abundant hydroxyl groups on the wood surface through hydrogen bonds or even covalent bonds, constructing a robust physicochemical bond. Simultaneously, its non-polar terpene segments achieve excellent compatibility with the plant-based film-forming resin system of this invention, thereby establishing a strong "molecular bridge" between the organic coating and the wood substrate. This significantly enhances the adhesion of the entire coating system to the wood substrate and ensures long-term stability. The molecular weight of this adhesion promoter is precisely controlled to allow it to effectively penetrate the microporous structure of the wood surface while forming a stable colloidal dispersion with the coating system.

[0070] Furthermore, the plant-based waterproofing agent is a nanoscale modified silane emulsion, and its preparation process is carefully designed to achieve a synergistic effect of highly effective waterproofing and excellent transparency. Its core preparation steps include: First, nano-silica particles extracted from rice husk ash, with an average particle size typically controlled between 10 and 50 nanometers. These nano-silica particles undergo surface esterification modification, for example, by reacting with plant-derived fatty acids such as salicylic acid, stearic acid, or oleic acid under the action of a catalyst, causing the surface hydroxyl groups to be replaced by hydrophobic groups, thereby preparing nano-silica particles with good hydrophobicity. This step ensures the compatibility of the nanoparticles with the subsequent silane system and imparts hydrophobic properties. Second, vinyltrimethoxysilane and octyltriethoxysilane are mixed at a specific molar ratio (e.g., 1:0.5 to 1:1.5). Partial hydrolysis and condensation reactions are carried out under gentle stirring conditions in the presence of a dilute acid (such as acetic acid or hydrochloric acid, concentration 0.01M to 0.1M) as a catalyst. The reaction temperature is typically controlled between 25°C and 40°C, and the reaction time is 2 to 4 hours, thereby forming an oligomeric silane prepolymer with a specific molecular weight and silanol content. This prepolymer exhibits both reactivity and a certain degree of hydrophobicity. Finally, the hydrophobic nano-silica particles prepared above are mixed with the oligomeric silane prepolymer at a mass ratio of 1:2 to 1:5 and emulsified under high-speed shear (e.g., using a high-shear emulsifier at a speed of 5000 rpm to 10000 rpm). Simultaneously, appropriate amounts of vegetable oil-based emulsifiers (such as castor oil polyoxyethylene ether, lecithin, or sucrose fatty acid esters) and stabilizers are added to ensure the formation of a nanoscale silane emulsion with excellent stability and permeability. The particle size of this emulsion is typically controlled below 100 nanometers to ensure its transparency in the coating. When applied to the wood surface, this plant-based waterproofing agent forms a dense, nano-scale hydrophobic protective layer that effectively prevents the intrusion of external liquid moisture. Simultaneously, its breathable yet waterproof properties allow moisture to slowly escape from the wood, preventing moisture buildup. By reducing fluctuations in wood moisture content, this waterproofing agent indirectly reduces the probability of water-induced red staining, extending the coating's protective period.

[0071] Furthermore, the plant-based plasticizer / film-forming agent is triethyl citrate. This plasticizer is typically prepared by esterification of citric acid with industrial-grade anhydrous ethanol in the presence of an acid catalyst (such as sulfuric acid or p-toluenesulfonic acid). Typical reaction conditions are as follows: citric acid, ethanol, and catalyst are added to a reaction vessel, and the reaction is carried out under stirring and heating conditions (e.g., 80°C to 100°C). Water generated during the reaction is removed by an azeotropic method to shift the reaction equilibrium towards the product. After the reaction, high-purity triethyl citrate is obtained through neutralization, washing, and distillation purification. Triethyl citrate, as an environmentally friendly plasticizer, exhibits excellent biodegradability and low toxicity. Its addition to coating systems can effectively lower the glass transition temperature (Tg) of the film-forming resin, thereby significantly improving the flexibility, crack resistance, and impact resistance of the coating film, making it less prone to cracking when wood undergoes minor deformation due to temperature or humidity changes. At the same time, it also helps improve the leveling properties of the coating, ensuring that the coating film can be spread evenly during the construction process, forming a smooth, dense and highly transparent coating.

[0072] Furthermore, the plant-based preservative and antifungal agent is a composite essential oil microemulsion. This microemulsion is typically prepared by precisely proportioning composite essential oils extracted from plants such as thyme, cinnamon, clove, and tea tree (whose main active components are phenolic compounds such as thymol and eugenol, and aldehyde compounds such as cinnamaldehyde) with plant-derived surfactants (such as lecithin, polyoxyethylene hydrogenated castor oil, and sucrose fatty acid esters). In a container equipped with high-speed shearing or ultrasonic emulsification equipment, a high-speed emulsification process is performed at 20°C to 40°C for 10 to 20 minutes to form a stable microemulsion with a particle size of less than 50 nanometers. This microemulsion, through nanoscale uniform dispersion, ensures that its active ingredients can fully penetrate and evenly distribute within the coating. The various phenolic and aldehyde compounds it contains have broad-spectrum antibacterial and antifungal activities, which can effectively inhibit various molds (such as Penicillium and Aspergillus) and decay fungi (such as brown rot and white rot) that are prone to grow in wood in humid and warm environments, thereby preventing microorganisms from causing biodegradation of the coating and wood substrate, and ensuring the long-term stability and integrity of the coating system and wood heritage.

[0073] Furthermore, the plant-based defoaming stabilizer is a modified castor oil-based polysiloxane. The preparation process of this defoamer typically involves multiple chemical reactions. First, castor oil is modified by esterification or etherification using an ethylene oxide / propylene oxide block copolymer to obtain a castor oil derivative with a certain hydrophilic-lipophilic balance (HLB value). Subsequently, this castor oil derivative is graft copolymerized with polymethylsiloxane (e.g., hydroxyl-terminated polydimethylsiloxane) in the presence of a catalyst (such as a strong acid or strong base) through condensation or addition reactions to obtain the modified castor oil-based polysiloxane. Its molecular structure contains lipophilic siloxane segments and hydrophilic castor oil derivative segments, endowing it with unique surface activity and spreading ability. During high-speed dispersion, grinding, and subsequent application in coating preparation, this defoamer can rapidly migrate to the surface of bubbles, reduce surface tension, promote bubble aggregation and collapse, thereby effectively disintegrating bubbles generated in the coating system. This ensures the smoothness, uniformity, and high transparency of the final coating, avoiding surface defects caused by air bubbles. Simultaneously, its stable chemical structure helps improve the storage stability of the coating system, preventing delamination, sedimentation, or excessive air bubble formation before use.

[0074] Furthermore, the plant-based crosslinking accelerator is an environmentally friendly polyamine derivative. Its preparation can be achieved by chemically modifying bio-polyamines (such as spermine, spermidine, putrescine, and cadaverine) extracted from seaweed, yeast, or other biomass. For example, these bio-polyamines can be subjected to addition reactions with ethylene oxide or propylene oxide, or amidation reactions with fatty acids, to reduce their volatility and toxicity, and to give them higher reactivity and better compatibility. The modified polyamine derivative molecules contain multiple active amino groups, which can undergo rapid crosslinking reactions with specific functional groups (such as epoxy groups, carboxyl groups, or carbon-carbon double bonds of acrylates) in the plant-based film-forming resin system of this invention under room temperature or low-temperature conditions (such as 20°C to 40°C). This crosslinking accelerator, by forming chemical bonds, connects the resin macromolecular chains into a three-dimensional network structure, thereby accelerating the curing speed of the coating film, significantly improving the hardness, wear resistance, scratch resistance, and water and chemical corrosion resistance of the coating film, and further enhancing the long-term protective capability of the coating.

[0075] Furthermore, the deionized water is an essential pure solvent in the coating system of this invention. Its preparation typically employs a multi-stage purification process. First, reverse osmosis (RO) membrane separation technology removes most of the ions, particulate matter, organic matter, and microorganisms from the water. Subsequently, the RO effluent is introduced into anion and cation exchange resin columns for further treatment. Through the adsorption and exchange action of the resin, residual trace ions are thoroughly removed, reducing the water conductivity to below 0.055 μS / cm. Finally, the deionized water is filtered through a microporous membrane with a pore size of 0.22 micrometers or smaller to remove any remaining particles and microorganisms. Ensuring the high purity of the deionized water aims to provide a solvent system free of any soluble ions, organic matter, and microorganisms for coating preparation, thereby effectively avoiding potential coating stability problems, interfacial side reactions, and adverse effects on coating transparency and performance caused by impurities.

[0076] The present invention provides a method for preparing a plant-based transparent flame-retardant coating, aiming to ensure that all components are fully dispersed, uniformly ground, and stably mixed through a precisely controlled process, thereby obtaining a high-performance transparent flame-retardant coating. The preparation method includes the following key steps: The first step, synthesis and microencapsulation of plant-based phosphorus-nitrogen flame retardants: This step has been described in the detailed description of plant-based phosphorus-nitrogen flame retardants above. Strict control of the synthesis conditions for phosphorus and nitrogen sources is required, including temperature, reaction time, catalyst dosage, and purification steps, to ensure the purity and activity of the product. The microencapsulation process is of paramount importance, requiring precise control of the dispersion droplet acceleration rate, stirring intensity, capsule wall material concentration, and solvent evaporation rate to ensure the acquisition of microencapsulated flame retardant powder with uniform particle size, complete encapsulation, and stable wall material. Product quality should be rigorously monitored through particle size analysis (e.g., dynamic light scattering), morphology analysis (e.g., scanning electron microscopy), and encapsulation rate determination.

[0077] The second step is the synthesis of plant-based film-forming resins: This step has already been described in the detailed description of plant-based film-forming resins above. The synthesis of cashew phenol-modified epoxy acrylate resin and modified cellulose ether should be carried out separately, with strict control over key parameters such as reaction temperature, time, catalyst dosage, monomer ratio, and acid value. The two resins obtained need to be characterized separately (e.g., infrared spectroscopy, nuclear magnetic resonance), determined by molecular weight (e.g., gel permeation chromatography), and tested for physicochemical properties (e.g., acid value, viscosity, solid content) to ensure they meet the design specifications. Subsequently, the two resins are mixed and stirred at a specified mass ratio at a specific temperature (e.g., 50℃ to 60℃) for 2 hours to ensure complete miscibility and uniform compounding, forming a stable composite resin with a viscosity within a specific range.

[0078] The third step is the preparation of plant-based interface stabilizers: This step has already been described in the detailed description of plant-based interface stabilizers above. The extraction, purification, and nanoemulsification of natural phenolic free radical scavengers, as well as the synthesis and nano-powder preparation of plant-based chelating buffer salts, all require strict control of process parameters, such as extraction pressure, temperature, homogenization pressure, neutralization pH, and drying conditions, to ensure that their activity and particle size meet the requirements. Product quality should be evaluated by determining the phenolic content using high-performance liquid chromatography (HPLC), measuring particle size using dynamic light scattering, testing pH buffering capacity, and testing metal ion chelating capacity.

[0079] The fourth step is the overall preparation of the plant-based transparent flame-retardant coating: This is a multi-stage, refined mixing and dispersion process that must be carried out in a clean production environment.

[0080] The specific process is as follows: The first stage is the dispersion phase: 70% to 80% of the total deionized water is accurately measured and added to a high-speed dispersion vessel equipped with a high-efficiency dispersion disc. While stirring, the previously prepared plant-based film-forming resin, the nano-emulsified and powdered plant-based interface stabilizer, the plant-based antioxidant and weather-resistant agent, the plant-based adhesion promoter, and the plant-based plasticizer / film-forming agent are added slowly and sequentially at a speed of 1500 to 2500 rpm. After each component is added, it should be thoroughly stirred for 5 to 10 minutes to ensure initial wetting and uniform dispersion. The entire dispersion phase lasts 30 to 60 minutes to allow the components to form a stable pre-dispersion system in the aqueous phase, preventing agglomeration. During dispersion, the system temperature should be monitored to ensure it does not exceed 35°C.

[0081] The next stage is the grinding process: the pre-dispersed system is transferred to a horizontal sand mill or a vertical bead mill via pump. Then, the microencapsulated plant-based phosphorus-nitrogen flame retardant and plant-based carbon source prepared earlier are added. High-hardness, high-wear-resistance zirconia beads are used as the grinding media, with a particle size typically selected from 0.8 mm to 1.2 mm. Grinding is carried out at a linear speed of 2500 rpm to 3500 rpm under continuous cooling circulation. The grinding process lasts for 2 to 4 hours, with periodic sampling and analysis using a laser particle size analyzer until the average particle size of the solid particles reaches less than 500 nanometers. This fine grinding aims to ensure that the flame retardant and carbon source achieve nanoscale or submicron-level dispersion in the coating system, thereby guaranteeing the excellent transparency and uniformity of the final coating film. During the grinding process, the system temperature must be strictly controlled, ensuring it never exceeds 40°C, to prevent thermal degradation of components or premature rupture of microcapsules.

[0082] Next is the formulation stage: After filtering the ground slurry (e.g., using a 300-mesh filter) to remove grinding media and coarse particles, it is transferred to a formulation vessel equipped with a low-speed stirrer. Then, the remaining deionized water, plant-based waterproofing agent, plant-based preservative and mildew-preventing agent, plant-based defoaming stabilizer, and plant-based crosslinking accelerator are precisely measured and added. The mixture is stirred continuously at 500 to 800 rpm for 30 to 60 minutes to ensure all components are thoroughly mixed and homogeneous, forming a finished coating with specific viscosity and rheological properties. During the formulation process, preliminary testing of key indicators such as viscosity, pH value, and solids content should be performed.

[0083] Finally, curing and filtration are performed: the prepared coating is left to cure at 25°C for 12 to 24 hours. This curing process allows for sufficient molecular alignment and physicochemical reactions between the components, thereby improving the coating's storage stability and application performance. After curing, the coating is precisely filtered through a 100-200 mesh screen to remove any possible micro-impurities, gel particles, or incompletely dispersed agglomerates, ultimately yielding a clear, uniform, and sediment-free plant-based transparent flame-retardant coating. The final product undergoes comprehensive quality testing, including appearance, transparency, viscosity, density, solids content, pH value, storage stability, flame retardant properties, and adhesion.

[0084] The application method of the plant-based transparent flame-retardant coating provided by this invention is designed with a systematic and multi-layered protection process, taking into account the special characteristics and complexity of wooden architectural heritage, aiming to maximize the realization of the cultural relic protection concept of "restoring the old as it was". The process includes: The first step is the testing and evaluation of the wood substrate: Before applying the coating, a comprehensive and detailed non-destructive testing and evaluation of the target old wood substrate is essential. First, a non-contact infrared thermometer is used to accurately measure the surface temperature of the wood, combined with a portable wood moisture meter (such as a resistance or dielectric type) to measure the moisture content at different depths within the wood, with an accuracy within ±1%. Then, an ultrasonic flaw detector or elastic wave testing equipment is used to scan the wood using a dot matrix method to detect the presence of cavities, decay, termite infestation, or other structural defects, and a defect distribution map is created. Simultaneously, the wood surface is visually inspected, recording its original color, texture characteristics, degree of damage, and the presence of weathering, cracking, deformation, discoloration (especially early signs of reddening), and microbial infestation. Based on these detailed test results, a personalized pretreatment and coating application plan is developed for each wooden component, or even different areas of the same wooden component, including moisture content adjustment, surface repair, primer application amount, and the number of intermediate coats, to ensure the accuracy and effectiveness of the protective measures.

[0085] The second step is cleaning the wood surface and using the "open-mouth drainage method": First, perform preliminary cleaning: Use a high-pressure air gun (pressure controlled between 0.2MPa and 0.4MPa) and an ultra-fine soft brush (bristle length not less than 50mm, diameter not greater than 0.15mm) to gently and thoroughly remove dust, loose fibers, cobwebs, and other debris from the wood surface. For stubborn stains, use a neutral pH (pH 6.5 to 7.5) plant-based cleaner (such as cocamidopropyl betaine or glucoside cleaner) with a microfiber cloth or natural sponge for gentle wiping. After cleaning, wipe repeatedly with a clean, damp cloth and dry with a low-pressure air gun. Avoid using any strong acid or alkali cleaners or rough tools to prevent abrasion or chemical damage to the wood surface.

[0086] followed by "Live Drainage Method" For damp, old wood with a moisture content exceeding 18% as identified in the aforementioned assessment, this invention employs a unique "open-mouth drainage method" for gentle, non-destructive drying. The core of this method lies in accelerating the natural dissipation of moisture from within the wood through precisely controlled micropores. Specifically, in concealed areas of the wooden components, such as mortise and tenon joints, shaded sides, or less noticeable lower areas, micropores with a diameter of 5 mm to 8 mm and a depth of two-thirds to three-quarters of the wood's thickness are selectively drilled using a precision drill bit. The spacing between the pores should be determined based on the wood's size and moisture level, typically 20 cm to 30 cm. These pores act as "openings," accelerating the diffusion of bound and free water from the wood's interior through the wood's own capillary action and natural air convection. To further improve drainage efficiency, a small amount of highly hygroscopic plant fiber material (such as corn starch fiber, cotton cellulose, or hemp fiber) or inert desiccant particles (such as silica gel microspheres) can be filled inside the pores. These materials swell or change color after absorbing water, serving as indicators of moisture removal. Simultaneously, miniature ventilation devices, such as low-power, low-speed (0.5 m / s to 1.5 m / s) axial flow fans or circulating fans, are installed 30 to 50 centimeters above the wood surface to create gentle, uniform airflow, promoting moisture evaporation from the pores and the wood surface, and preventing localized excessively rapid drying. Throughout the drying process, the wood moisture content is continuously monitored using a portable wood moisture meter until it uniformly drops to the ideal range of 8% to 15%. This method effectively avoids the stress damage, cracking, deformation, and internal structural degradation risks caused by traditional high-temperature drying of old wood, maximizing the protection of the wood's original physical structure and cultural value.

[0087] The third step is the application of a plant-based wood interface stabilizing primer: After the wood substrate has been thoroughly cleaned and its moisture content adjusted to meet construction requirements, the aforementioned prepared plant-based transparent flame-retardant coating is diluted, and the solid content of the plant-based interface stabilizer is precisely adjusted to increase to 15% to 20%. This diluted coating serves as a primer. The plant-based wood interface stabilizer primer is applied evenly and in a thin layer to the wood surface using a soft brush, roller, or low-pressure spraying (spray gun pressure controlled at 0.1 MPa to 0.2 MPa). The coating thickness must be strictly controlled between 30 and 50 micrometers to ensure the formation of a uniform and dense film. After coating, surface drying is allowed for 30 to 60 minutes and complete drying for 4 to 6 hours under ambient temperatures of 20°C to 25°C and relative humidity of 50% to 70%. The natural phenolic free radical scavenger in this primer can rapidly penetrate to the wood surface, effectively passivating active phenolic substances, lignin degradation products, and cellulose degradation products in the wood, interrupting the oxidative polymerization reaction chain by capturing free radicals. Meanwhile, the plant-based chelated buffer salt can penetrate deep into the micropores of wood, stabilize the interface pH to near neutral, and efficiently chelate transition metal ions such as iron and copper that may be present in the wood, thereby inhibiting the red staining reaction at both the molecular and ionic levels. This primer layer provides a stable interface with no risk of red staining for the subsequent flame-retardant intermediate coating.

[0088] Step 4: Applying the plant-based transparent intumescent flame-retardant intermediate coat: After the primer is completely dry and no abnormalities (such as signs of reddening) are confirmed, the aforementioned undiluted plant-based transparent flame-retardant coating is applied as the intermediate coat, using a brush or roller to apply multiple layers. The thickness of each coat should be controlled between 80 and 120 micrometers, ensuring uniformity and no sagging. The interval between coats should be 6 to 8 hours to ensure the previous coat is fully dry and has reached sufficient strength before applying the next coat, thus avoiding coating defects. The total number of coats depends on the flame-retardant rating and type of wood, typically 2 to 3 layers. The final total thickness should be controlled between 200 and 300 micrometers. In this intermediate coat layer, microencapsulated plant-based phosphorus and nitrogen flame retardants and plant-based carbon sources are evenly distributed. In the event of a fire, the microcapsule walls will rupture rapidly, releasing phosphoric acid and nitrogen sources that, in conjunction with the plant-based carbon source, catalyze a dehydration and carbonization reaction in the wood and the coating itself, forming a dense, high-expansion, and strongly adhesive intumescent carbonized layer. This carbonized layer effectively isolates heat and oxygen from being transferred into the wood substrate, significantly slowing the spread of flames and thus greatly improving the fire resistance limit and flame retardant rating of the wood.

[0089] Step 5: Applying the plant-based weather-resistant protective topcoat: After the intermediate coat has fully cured (typically requiring 24 to 48 hours, depending on environmental conditions), a protective topcoat is applied. The plant-based transparent flame-retardant coating described in this invention is diluted again, and the proportions are finely adjusted to increase the solid content of the plant-based antioxidant and weather-resistant agent and the plant-based waterproofing agent to 10% to 15% as the final protective topcoat. The topcoat is applied evenly and thinly to the surface of the flame-retardant intermediate coat using low-pressure spraying (spray gun pressure controlled at 0.15 MPa to 0.25 MPa, with uniform spray width) or fine brushing, with the coating thickness precisely controlled to 40 to 60 micrometers. After the topcoat is applied, it is cured at an ambient temperature of 20°C to 25°C and a relative humidity of 50% to 70%, typically requiring 24 to 48 hours, to ensure complete cross-linking of the coating and the formation of its final properties. This topcoat layer forms a tough, wear-resistant, highly transparent protective layer with excellent weather resistance and water resistance, effectively resisting damage from external environmental factors such as ultraviolet radiation, acid rain erosion, humidity changes, and microbial invasion. This topcoat layer not only extends the service life of the entire coating system, but more importantly, it maintains the original aesthetic effect of the wood for a long time, ensuring that the concept of "restoring the old as it was" is realized in historical buildings.

[0090] To further illustrate the technical effects and performance of the present invention, specific embodiments, comparative examples, and experimental data are provided below.

[0091] Example 1: Preparation and Application of Plant-Based Transparent Flame-Retardant Coatings In this embodiment, following the mass percentage range and detailed preparation method steps given in the foregoing invention, an old pine board with typical reddening tendency was prepared and applied.

[0092] Paint components (by weight): Plant-based film-forming resin: 35% Plant-based phosphorus and nitrogen flame retardant: 22% Plant-based carbon source: 10% Plant-based interface stabilizer: 6% Plant-based antioxidant and weather-resistant agents: 4% Plant-based adhesion promoter: 2% Plant-based waterproofing agent: 2% Plant-based plasticizer film-forming agent: 2% Plant-based preservative and anti-mildew agent: 1% Plant-based defoaming stabilizer: 0.8% Plant-based cross-linking accelerator: 0.7% Deionized water: 14.5% Specific preparation process: Microencapsulation of plant-based phosphorus and nitrogen flame retardants: Synthesis of PPAE from phosphorus source: 100 parts of phytic acid and 60 parts of phosphoric acid were reacted at 160℃ under nitrogen protection for 4 hours to obtain PPAE.

[0093] Synthesis of nitrogen-based bio-based polyamide derivatives: 90 parts of plant protein hydrolysate and 50 parts of urea were mixed with 3 parts of zinc acetate catalyst and reacted at 135°C for 5 hours.

[0094] Microencapsulation: PPAE and bio-based polyamide derivatives were mixed at a ratio of 1.2:1 and dispersed at high speed. The mixture was then interfacially polymerized with a polylactic acid (PLA) solution in dichloromethane (PLA content 12%) at 30°C for 7 hours. After solvent evaporation, washing, and vacuum drying, microcapsule powder with an average particle size of 200 nm was obtained.

[0095] Synthesis of plant-based film-forming resins: Cashew phenol-modified epoxy acrylate resin: 100 parts cashew phenol were reacted with 60 parts sodium hydroxide aqueous solution and 100 parts epichlorohydrin at 85℃ for 4 hours to obtain cashew phenol glycidyl ether. Then, it was reacted with 45 parts acrylic acid at 110℃ under triphenylphosphine catalysis for 6 hours, reducing the acid value to 4 mg KOH / g.

[0096] Modified cellulose ether: 100 parts of plant cellulose pulp were alkalized with 1200 parts of isopropanol and 60 parts of sodium hydroxide aqueous solution at 45°C for 1 hour. A mixture of 50 parts of ethylene oxide and 40 parts of propylene oxide was added dropwise, and the reaction was carried out at 75°C for 4 hours. After neutralization, washing, and drying, hydroxyethyl hydroxypropyl cellulose ether was obtained.

[0097] Preparation of composite resin: Cashew phenol modified epoxy acrylate resin and modified cellulose ether are stirred at 55℃ for 2 hours at a mass ratio of 1.8:1.

[0098] Preparation of plant-based interface stabilizers: Phenolic free radical scavenger nanoemulsion: 100 parts of grape seed pomace were extracted with supercritical carbon dioxide, concentrated and dissolved in ethanol, and homogenized with lecithin and polysorbate 80 at 120 MPa 7 times to prepare a nanoemulsion with a particle size of 30 nanometers.

[0099] Chelated buffer salt powder: 100 parts of citric acid and quinoa polypeptide solution were neutralized in equimolar amounts and spray-dried to obtain a powder with a particle size of 80 nanometers.

[0100] Preparation of overall coating: Dispersion: Add 75% deionized water to the reactor, then add the composite resin, interface stabilizer nanoemulsion, chelated buffer salt powder, antioxidant and weather-resistant agent, adhesion promoter, and plasticizer / film-forming agent in sequence. Disperse at 1800 rpm for 45 minutes.

[0101] Grinding: The dispersion was transferred to a sand mill, and the microencapsulated flame retardant and carbon source were added. It was then ground for 3 hours at a linear speed of 3000 rpm using 1.0 mm zirconia beads until the particle size D90 was less than 400 nm.

[0102] Preparation: Add the remaining deionized water, waterproofing agent, anti-corrosion and anti-mildew agent, defoaming stabilizer, and cross-linking accelerator to the ground slurry. Stir at 600 rpm for 45 minutes.

[0103] Curing and filtration: The coating is left to stand at 25°C for 18 hours to cure, and then filtered through a 150-mesh filter.

[0104] Construction and application process: Select an old pine board (30cm x 15cm x 2cm) with slight decay and a moisture content as high as 22%.

[0105] Substrate treatment: An infrared thermometer showed a surface temperature of 23°C, while a moisture meter showed a localized moisture content as high as 22%. Visual inspection revealed slight weathering and localized discoloration. Three 6mm diameter, 1.5cm deep "open-ended" holes were drilled 15cm apart on the shaded side of the wooden board, and filled with cornstarch fiber. A miniature fan was installed at a speed of 1.0m / s, and drying was continued for 72 hours until the moisture content uniformly decreased to 12%. The surface was cleaned with a neutral plant-based detergent.

[0106] Primer application: Dilute the above coating and adjust the interface stabilizer solid content to 18% to serve as the primer. Apply a uniform layer with a soft brush, approximately 40 micrometers thick. Surface dry for 45 minutes at 23°C and 60% relative humidity, then allow to fully dry for 5 hours.

[0107] Intermediate coat application: After the primer has fully dried, apply two coats of the original paint as the intermediate coat using a roller coating method. Each coat is approximately 100 micrometers thick. Allow 7 hours between coats. The total intermediate coat thickness is 200 micrometers.

[0108] Topcoat application: After the intermediate coat has fully cured (24 hours), dilute the paint and adjust the solid content of the antioxidant, weather-resistant agent, and waterproofing agent to 12% as the topcoat. Apply one layer using low-pressure spraying, with a thickness of approximately 50 micrometers. Curing time is 48 hours at 23°C and 60% relative humidity.

[0109] Comparative Example 1: Plant-based transparent flame-retardant coating without interface stabilizers and microencapsulated flame retardants The coating formulation in this comparative example is similar to that in Example 1, but the plant-based interface stabilizer is omitted, and the plant-based phosphorus-nitrogen flame retardant is directly added as a mixed powder of unmicroencapsulated PPAE and bio-based polyamide derivatives. Other components and preparation and application processes are consistent with those in Example 1.

[0110] Paint components (by weight): Plant-based film-forming resin: 35% Non-microencapsulated plant-based phosphorus and nitrogen flame retardant: 22% (a mixture of PPAE and bio-based polyamide) Plant-based carbon source: 10% Plant-based interface stabilizers-free: 0% Plant-based antioxidant and weather-resistant agents: 4% Plant-based adhesion promoter: 2% Plant-based waterproofing agent: 2% Plant-based plasticizer film-forming agent: 2% Plant-based preservative and anti-mildew agent: 1% Plant-based defoaming stabilizer: 0.8% Plant-based cross-linking accelerator: 0.7% Deionized water: 20.5% Specific preparation process (key differences): Phosphorus-nitrogen flame retardants: PPAE powder and bio-based polyamide derivative powder are directly mixed without microencapsulation.

[0111] Dispersion and grinding: During the dispersion and grinding stage, the unmicroencapsulated phosphorus-nitrogen flame retardant mixed powder was directly added. Other dispersion, grinding, preparation, curing and filtration steps were the same as in Example 1, but due to the large particle size of the flame retardant and its tendency to agglomerate, the grinding time was extended to 4.5 hours, and the D90 barely reached 700 nanometers.

[0112] Free of interfacial stabilizers: The addition of plant-based interfacial stabilizers is omitted during the preparation process.

[0113] Construction and application process: An old pine board of the same batch, size, and condition as in Example 1 was selected. The substrate treatment process (open-mouth drainage method) was exactly the same as in Example 1, and the moisture content was also reduced to 12%. The coating process (number of layers, thickness, and interval of primer, intermediate coat, and topcoat) was consistent with that in Example 1, but the primer formulation did not contain an interface stabilizer.

[0114] Experimental comparison and result analysis: A series of performance tests were conducted on the wood boards coated in Example 1 and Comparative Example 1, as well as a blank old pine board without any treatment (as a control group), including red color inhibition rate, flame retardancy (oxygen index LOI, vertical burning test), coating adhesion, weather resistance (gloss retention rate) and water resistance.

[0115] 1. Redness inhibition rate assessment: Red stain inhibition rate was measured using a colorimeter (Chroma Meter CR-400) on the wood surface before and after coating, and after aging. a b The value is used to calculate the chromaticity difference (ΔE). L Indicates brightness, a Indicates red-green hue, b Indicates the degree of yellow-blue. Redness is mainly manifested in a. A significant increase in the value. The formula for calculating the redness inhibition rate is: Inhibition rate = (ΔE_control group - ΔE_sample group) / ΔE_control group × 100%.

[0116] Control group (untreated wood panels): After being placed in a humid environment for 7 days, ΔE_control group was 15.2, a The value increased from +3.5 to +11.8.

[0117] Comparative Example 1: After coating, the wood surface showed obvious red discoloration after 7 days in a humid environment. The ΔE_sample group was 8.7, a The value increased from +3.8 to +8.9. The inhibition rate was approximately 42.7%.

[0118] Example 1: After coating, the wood surface was placed in a humid environment for 7 days, and almost no red discoloration occurred. The color was basically the same as the original wood. ΔE_sample group was 1.5, a The value only increased from +3.7 to +4.1. The inhibition rate was as high as 89.9%.

[0119] This result clearly indicates that the present invention, through the synergistic effect of plant-based interface stabilizers (natural phenolic free radical scavengers and plant-based chelated buffer salts) and the physical isolation of the active components by microencapsulated flame retardants, can fundamentally inhibit the reddening phenomenon of old, damp wood after coating, thereby preserving the original color and texture of the wood to the greatest extent. Comparative Example 1, lacking interface stabilizers and microencapsulation protection, saw the active components in direct contact with the wood, resulting in significant reddening.

[0120] 2. Flame retardant performance test: Flame retardant performance is evaluated by limiting oxygen index (LOI, according to GB / T 2406.2-2009) and vertical burning test (according to GB / T 8626-2007).

[0121] Control group (untreated wood boards): LOI was 21.5%, and the vertical burning test result showed rapid burnout with no flame retardant effect.

[0122] Comparative Example 1: LOI is 32.1%. The vertical burning test results show burning dripping, and the char layer is thin and uneven, accompanied by some smoke. It is judged as B2 level.

[0123] Example 1: The LOI was as high as 40.3%. The vertical combustion test results showed rapid expansion to form a dense char layer with no dripping, effective suppression of flame spread, self-extinguishing without open flame, and very little smoke. It was rated as B1.

[0124] The coating of Example 1 rapidly expands at high temperatures to form a dense and stable char layer, effectively isolating heat and oxygen, and significantly improving the fire resistance limit and flame retardant rating of the wood. The microencapsulated flame retardant ensures that the flame retardant components can be released efficiently and in a concentrated manner during a fire, while the plant-based carbon source provides sufficient char precursors, resulting in a char layer expansion ratio and density superior to Comparative Example 1.

[0125] 3. Coating adhesion test: Adhesion was tested using the cross-cut adhesion test (according to GB / T 9286-1998) and rated from 1 to 6, with 1 being the best adhesion.

[0126] Control group (no coating): Not applicable.

[0127] Comparative Example 1: Adhesion grade is 2, with a small amount of paint film peeling off in the gridded areas.

[0128] Example 1: The adhesion level is 1, the edges of the coating in the grid area are smooth, and there is no peeling.

[0129] Plant-based adhesion promoters play a key role in this invention, and their molecular bridging effect significantly enhances the adhesion between the coating and the wood substrate.

[0130] 4. Weather resistance test: Weather resistance was evaluated by assessing the gloss retention rate of the coating after an accelerated aging test (according to GB / T 1865-2009, xenon lamp aging for 1000 hours). The original gloss level was set to 100%.

[0131] Control group (untreated wood boards): After aging, the surface showed obvious cracks and discoloration, and had no gloss.

[0132] Comparative Example 1: After aging for 1000 hours, the coating showed slight yellowing and fine cracks, with a gloss retention rate of 75%.

[0133] Example 1: After aging for 1000 hours, the coating film maintained excellent transparency and gloss, with no obvious yellowing or cracking, and the gloss retention rate reached 92%.

[0134] Plant-based antioxidant weather-resistant agents (modified tocopherol and plant polysaccharide crosslinkers) effectively resist ultraviolet radiation and oxidative degradation, significantly extending the service life of the coating and maintaining its aesthetic effect.

[0135] 5. Water resistance test: Water resistance was evaluated by the changes in the appearance of the coating (bubbling, loss of gloss, softening) after immersion in water (according to GB / T 1733-1993, immersion in deionized water for 24 hours).

[0136] Control group (untreated wood boards): The wood boards absorbed water and swelled, the surface became rough, and the color darkened.

[0137] Comparative Example 1: The coating surface showed slight loss of gloss and softening, with tiny bubbles at the edges.

[0138] Example 1: The coating surface showed no changes, maintaining its original transparency and gloss, with no bubbling or softening.

[0139] The nano-silane emulsion in the plant-based waterproofing agent forms a dense hydrophobic layer on the coating surface, effectively preventing water penetration and exhibiting excellent water resistance.

[0140] Comprehensive experimental data table: The comparative data from the above embodiments and comparative examples clearly demonstrate that the plant-based transparent flame-retardant coating for the preservation of timber-framed architectural heritage provided by this invention exhibits superior comprehensive performance in key indicators such as red stain inhibition, flame retardancy, adhesion, weather resistance, and water resistance. In particular, it achieves a significant breakthrough in inhibiting the red staining of old, damp wood through the introduction of plant-based interface stabilizers and microencapsulated flame retardants, which is of great significance for the preservation of cultural relics adhering to the principle of "restoring the old as it was." Furthermore, its application of all-plant-based components gives it unparalleled advantages in terms of environmental friendliness and sustainability.

Claims

1. A plant-based transparent fire-retardant coating for the protection of ancient buildings, characterized by, The coating is composed of the following components in mass percentage: Plant-based film-forming resins: 25% to 45%; Plant-based phosphorus and nitrogen flame retardants: 15% to 30%; Plant-based carbon sources: 5% to 15%; Plant-based interface stabilizers: 3% to 10%; Plant-based antioxidants and weather-resistant agents: 2% to 8%; Plant-based adhesion promoter: 1% to 5%; Plant-based waterproofing agent: 1% to 5%; Plant-based plasticizers for film formation: 1% to 5%; Plant-based preservative and antifungal agent: 0.5% to 3%; Plant-based defoaming stabilizer: 0.5% to 2%; Plant-based cross-linking accelerator: 0.5% to 2%; Deionized water: Balance; The plant-based phosphorus-nitrogen flame retardant is a microencapsulated composite intumescent flame retardant; the plant-based interface stabilizer includes natural phenolic free radical scavengers and plant-based chelated buffer salts.

2. The plant-based transparent fire-retardant coating for the protection of ancient buildings according to claim 1, characterized in that, In the plant-based interface stabilizer: The natural phenolic free radical scavenger is a polyphenolic compound rich in proanthocyanidins, catechins and gallic acid, extracted from grape seeds or green tea using supercritical fluid extraction technology. The polyphenolic compound is dispersed in the coating system using nanoemulsion technology, and the particle size of the nanoemulsion is less than 50 nanometers. The plant-based chelated buffer salt is a complex formed by neutralizing an alkaline polypeptide extracted from quinoa with citric acid or oxalic acid, and the particle size of the complex is controlled at the nanoscale, less than 100 nanometers.

3. The plant-based transparent fire-retardant coating for the protection of ancient buildings according to claim 2, characterized in that, The plant-based phosphorus-nitrogen flame retardant is a microencapsulated composite intumescent flame retardant, which includes polymeric phytic acid phosphate as a phosphorus source and bio-based polyamide or polyguanidine salt derivatives as a nitrogen source. The polymeric phytic acid phosphate is formed by esterification and polymerization of phytic acid and phosphoric acid at 150°C to 170°C. The bio-based polyamide or polyguanidine salt derivative is generated by condensing plant protein hydrolysate with urea or guanidine salt at 130°C to 140°C. The PPAE and the bio-based polyamide or polyguanidine salt derivative are mixed at a mass ratio of 1:1 to 1.5:1 and then microencapsulated by interfacial polymerization or spray drying. The capsule wall material is selected from biodegradable polylactic acid or polyhydroxybutyrate. The average particle size of the microcapsules is controlled between 50 nanometers and 300 nanometers to physically isolate the flame retardant active component from the wood substrate.

4. The plant-based transparent fire-retardant coating for the protection of ancient buildings according to claim 3, characterized in that, The plant-based film-forming resin is a composite resin copolymerized from cashew phenol-modified epoxy acrylate resin and modified cellulose ether. The preparation process of the cashew phenol modified epoxy acrylate resin includes: etherifying cashew phenol with epichlorohydrin under alkaline conditions to obtain cashew phenol glycidyl ether, and then performing a ring-opening esterification reaction between the cashew phenol glycidyl ether and acrylic acid. The modified cellulose ether is prepared by reacting plant cellulose with hydroxyethyl etherifying agent and hydroxypropyl etherifying agent under the action of an alkaline catalyst to obtain hydroxyethyl hydroxypropyl cellulose ether with a specific degree of substitution. 5.The application method of the plant-based transparent fire-retardant coating for ancient building protection according to claim 1, characterized in that, Includes the following steps: (1) Wood substrate testing and evaluation: Surface temperature, internal moisture content, structural defects and surface condition of old wood substrates are tested and evaluated, and personalized pretreatment and construction plans are developed based on the results; (2) Wood surface cleaning and "open-mouth drainage method": The wood surface is initially cleaned; for damp old wood with a moisture content higher than 18%, the "open-mouth drainage method" is used for drying treatment. The method includes drilling micro holes with a diameter of 5 mm to 8 mm and a depth of two-thirds to three-quarters of the wood thickness in the concealed parts of the wood components. The holes are spaced 20 cm to 30 cm apart. The diffusion of moisture inside the wood is accelerated by natural convection and capillary action. The drainage efficiency can be further improved by filling the holes with hygroscopic plant fiber materials or inert desiccant particles. At the same time, a micro ventilation device is set up on the wood surface to form a gentle air flow. The moisture content is continuously monitored until it drops to 8% to 15%. (3) Application of plant-based wood interface stabilizer primer: dilute the coating described in claim 1 and adjust the concentration of the plant-based interface stabilizer to 15% to 20% (based on solid content) as primer, and apply it evenly to the cleaned and dried wood surface, with the coating thickness controlled to be 30 micrometers to 50 micrometers. (4) Application of plant-based transparent intumescent flame retardant intermediate coating: After the primer is completely dry, the undiluted coating of claim 1 is used as the intermediate coating and multiple layers are applied by brushing or rolling. Each layer is 80 to 120 micrometers thick, with an interval of 6 to 8 hours between layers. The total number of coating layers is 2 to 3, and the total thickness is controlled between 200 and 300 micrometers. (5) Application of plant-based weather-resistant protective topcoat: After the intermediate coat is fully cured, the coating described in claim 1 is diluted and the ratio of plant-based antioxidant weather-resistant agent and plant-based waterproof agent is adjusted so that the solid content in the topcoat is increased to 10% to 15% as a protective topcoat. The coating is uniformly applied to the surface of the flame-retardant intermediate coat and the coating thickness is controlled to be 40 micrometers to 60 micrometers.

6. The application method of the plant-based transparent fire-retardant coating for ancient building protection according to claim 5, characterized in that, In the "live-hole drainage method", the depth of the micro-holes is two-thirds to three-quarters of the thickness of the wood; the hygroscopic plant fiber material filling the holes is corn starch fiber or cotton cellulose, or the inert desiccant particles are silica gel microbeads; the micro-ventilation device is a low-power, low-speed axial flow fan or circulating fan, forming an airflow with a wind speed of 0.5m / s to 1.5m / s.

7. The application method of the plant-based transparent fire-retardant coating for ancient building protection according to claim 6, characterized in that, In the coating step of the plant-based wood interface stabilizing primer, the coating method is brushing, rolling, or low-pressure spraying, and the spray gun pressure of the low-pressure spraying is controlled at 0.1MPa to 0.2MPa; after coating, the coating is completely cured under the conditions of an ambient temperature of 20℃ to 25℃ and a relative humidity of 50% to 70%, which usually takes 24 hours to 48 hours. 8.The application method of the plant-based transparent fire-retardant coating for the protection of ancient buildings according to claim 7, characterized in that, In the coating step of the plant-based weather-resistant protective topcoat, the coating method is low-pressure spraying or brushing. The spray gun pressure of the low-pressure spraying is controlled at 0.15MPa to 0.25MPa. After coating, it is cured for 24 to 48 hours under the conditions of ambient temperature of 20℃ to 25℃ and relative humidity of 50% to 70%.

9. A method of preparing the plant-based transparent fire-retardant coating for the protection of ancient buildings according to claim 1, characterized by, Includes the following steps: (1) Synthesis and microencapsulation of plant-based phosphorus and nitrogen flame retardants: a. Phosphorus source synthesis: 90 to 110 parts of phytic acid and 50 to 70 parts of phosphoric acid are reacted at 150°C to 170°C under a nitrogen protective atmosphere for 3 to 5 hours to form polymeric phytic acid phosphate (PPAE). b. Nitrogen source synthesis: 80 to 100 parts of plant protein hydrolysate, 40 to 60 parts of urea, and 2 to 5 parts of zinc acetate are used as catalysts and reacted at 130°C to 140°C for 4 to 6 hours to generate bio-based polyamide derivatives. c. Microencapsulation: The PPAE obtained in step (1)a and the bio-based polyamide derivative obtained in step (1)b are mixed at a mass ratio of 1:1 to 1.5:1, dispersed at high speed, and then microencapsulated with a solution of polylactic acid (PLA) or polyhydroxybutyrate (PHB) as the capsule wall material. The average particle size of the microcapsules is in the range of 50 nanometers to 300 nanometers. (2) Synthesis of plant-based film-forming resins: a. Preparation of cashew phenol modified epoxy acrylate resin: Cashew phenol is etherified with epichlorohydrin under alkaline conditions, and then reacted with acrylic acid to obtain the resin. b. Preparation of modified cellulose ether: prepared by reacting plant cellulose with hydroxyethyl etherifying agent and hydroxypropyl etherifying agent under the action of an alkaline catalyst; c. Preparation of composite resin: The cashew phenol modified epoxy acrylate resin and the modified cellulose ether are mixed at a mass ratio of 1.5:1 to 2.5:1 and stirred at 50°C to 60°C for 2 hours. (3) Preparation of plant-based interface stabilizers: a. Extraction and nanoemulsification of natural phenolic free radical scavengers: Polyphenolic compounds were obtained by supercritical carbon dioxide extraction from grape seeds or green tea, and then nanoemulsified to prepare nanoemulsions with a particle size of less than 50 nanometers. b. Synthesis of plant-based chelated buffer salts: citric acid or oxalic acid is neutralized with basic peptides extracted from quinoa, and powder with a particle size of less than 100 nanometers is obtained by spray drying or freeze drying. (4) Overall preparation of plant-based transparent flame-retardant coating: a. Dispersion stage: Add 70% to 80% of the total deionized water to the dispersion vessel, and add plant-based film-forming resin, plant-based interface stabilizer, plant-based antioxidant and weather-resistant agent, plant-based adhesion promoter and plant-based plasticizer film-forming agent in sequence at a speed of 1500 rpm to 2500 rpm, and disperse for 30 minutes to 60 minutes. b. Grinding stage: Transfer the above dispersion system to a sand mill, add microencapsulated plant-based phosphorus and nitrogen flame retardant and plant-based carbon source, use 0.8 mm to 1.2 mm zirconia beads as grinding media, and grind at a linear speed of 2500 rpm to 3500 rpm for 2 to 4 hours under cooling circulation conditions to ensure that the solid particle size reaches less than 500 nanometers. c. Preparation stage: Transfer the ground slurry to a mixing tank, add the remaining deionized water, plant-based waterproofing agent, plant-based preservative and mildew-preventing agent, plant-based defoaming stabilizer and plant-based crosslinking accelerator, and stir at 500 rpm to 800 rpm for 30 to 60 minutes. d. Curing and filtration: Let the prepared paint stand at 25°C for 12 to 24 hours to cure, and then filter it through a 100 to 200 mesh filter.

Images