Modified polyurethane water-based functional coating and preparation process thereof

Abstract

The application discloses a modified polyurethane water-based functional coating and a preparation process thereof, and belongs to the technical field of water-based functional coatings. In the application, N,N-dimethylethanolamine is used as a blocking agent to block the isocyanate prepolymer, so that a latent type blocked isocyanate water-based polyurethane emulsion is prepared; lactide monomers, epoxy group grafted modified nano silicon dioxide and amino functionalized light stabilizer microcapsules are added into the emulsion; curing is carried out by using a two-stage process, that is, after preheating and water removal and film formation, 130-150 DEG C baking is carried out to trigger the deblocking of the DMEA blocked isocyanate, and the tertiary amine groups in the deblocked DMEA molecules immediately initiate the ring-opening polymerization of the premixed lactide monomers in the system in situ to generate polylactic acid segments which are chemically grafted to the crosslinked network. The application realizes the synchronous linkage of crosslinking and curing and polymer toughening, and the obtained coating has the characteristics of high crosslinking density, excellent flexibility and low internal stress.

Description

Technical Field

[0001] This invention relates to the field of waterborne functional coatings, and in particular to a modified polyurethane waterborne functional coating and its preparation process. Background Technology

[0002] Waterborne polyurethane coatings, using water as the dispersion medium, offer advantages such as low VOC emissions and environmental friendliness, making them widely used in wood coatings, automotive interior coatings, and architectural coatings. However, conventional one-component waterborne polyurethanes have a linear thermoplastic structure, lacking chemical crosslinking points between their molecular chains. This results in insufficient water resistance, solvent resistance, and hardness in the coating, limiting its application in high-performance coating fields. Introducing a crosslinking system is a key approach to improving the performance of one-component waterborne polyurethane coatings.

[0003] Blocked isocyanate technology is the mainstream approach for achieving latent crosslinking of one-component waterborne polyurethanes. Blocked isocyanates are compounds in which the -NCO groups are blocked by a blocking agent that cannot undergo deblocking at lower temperatures. These compounds do not polymerize at room temperature, but the -NCO groups can regenerate at high temperatures and crosslink with compounds containing active hydrogen. Dispersing blocked isocyanates in waterborne polyurethane emulsions allows them to remain stable during room temperature storage. During the baking and curing stage after application, the blocking agent deblocks, releasing active -NCO groups that react with hydroxyl and amino groups on the polyurethane molecular chain to form a crosslinked structure, significantly improving the coating's water resistance, chemical resistance, abrasion resistance, and mechanical properties. Given the increasing emphasis on environmental and energy issues, developing blocked isocyanates with lower deblocking temperatures and stable dispersion in water is a current research direction.

[0004] The choice of blocking agent has a decisive influence on the unblocking temperature. A blocked waterborne cationic isocyanate curing agent synthesized from butanone oxime, N,N-dimethylethanolamine (DMEA), and hexamethylene diisocyanate trimer (HDI trimer) has a particle size of 73.94 nm and an initial unblocking temperature of 130℃. Isocyanate curing agents blocked with 3,5-dimethylpyrazole (DMP) can achieve unblocking at 130-150℃.

[0005] In traditional blocked isocyanate systems, the role of the sealant is limited to protecting the isocyanate groups during storage, and it is released during thermosetting. The released sealant molecules remain in the coating, becoming inert byproducts and even performance hazards: small molecule sealants may slowly volatilize, causing coating shrinkage and cracking, or act as plasticizers, reducing coating hardness and heat resistance, or gradually migrate to the surface, causing surface defects. Although the industry has developed sealants with low desealing temperatures and low toxicity, the chemical fate of sealants as discarded after desealing remains unchanged.

[0006] On the other hand, although polyurethane coatings improve hardness and resistance to media after cross-linking and curing, they often face problems such as decreased flexibility, increased internal stress, and weakened adhesion to the substrate. This is because the formation of the cross-linked network restricts the movement of molecular chains, making the coating brittle. Traditional solutions involve adding plasticizers or toughening resins, but these have problems such as migration, volatilization, and poor compatibility with the matrix. Furthermore, physically blended toughening phases rely solely on weak interfacial forces with the matrix, making it difficult to achieve a durable and stable toughening effect.

[0007] Lactide is a cyclic dimer of lactic acid, derived from renewable biomass resources. It can undergo ring-opening polymerization under the action of a catalyst to produce polylactic acid (PLA). PLA exhibits good biodegradability and biocompatibility. While there are studies on introducing lactide into polyurethane systems, most methods involve first undergoing ring-opening polymerization to obtain polylactic acid diol, which is then reacted with isocyanate, or physical blending. The former represents a pre-polymerization followed by introduction approach, while the latter suffers from problems such as phase separation and uneven dispersion.

[0008] If we can break the traditional inert byproduct positioning of sealing agents and enable them to perform a second function after desealing—initiating lactide polymerization—linking the desealing process with the polymerization process, we can not only eliminate the negative effects of sealing agent residue, but also use the newly generated polylactic acid segments to toughen the coating in situ, achieving a technological breakthrough of one step, two reactions, and multiple effects; this has not been reported in this field. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of existing technologies by proposing a modified polyurethane waterborne functional coating and its preparation process. The core concept of this invention is to design a blocked isocyanate system that initiates polymerization immediately upon deblocking, using N,N-dimethylethanolamine (DMEA) as a blocking agent. This blocking agent immediately acts as a tertiary amine initiator / catalyst after deblocking, initiating the ring-opening polymerization of lactide, thus chemically linking the crosslinking reaction during the curing process with the polymerization reaction.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: A modified polyurethane waterborne functional coating comprises the following components: Component A: The raw materials for synthesizing latent blocked isocyanate-terminated waterborne polyurethane emulsions, comprising, by weight: 28-38 parts of isophorone diisocyanate (IPDI), as a hard segment monomer providing -NCO groups; 40-55 parts of polycarbonate diol (PCDL, Mn=2000), as a soft segment providing flexibility; 5-8 parts of dimethylolpropionic acid (DMPA), as a hydrophilic chain extender introducing carboxyl groups; 2-5 parts of 1,4-butanediol (BDO), as a small molecule chain extender; 6-15 parts of N,N-dimethylethanolamine (DMEA), as a blocking agent, and also as an initiator / catalyst for lactide polymerization; 4-7 parts of triethylamine (TEA), as a neutralizing agent to neutralize the carboxyl groups of DMPA; 180-250 parts of deionized water, as a dispersion medium; and 0.05-0.1 parts of dibutyltin dilaurate (DBTDL), as a prepolymerization catalyst.

[0011] Component B: Lactide-Film-Forming Aid Premix, comprising 5-20 parts L-lactide (derived from renewable lactic acid, produced by ring-opening polymerization to polylactic acid (PLA)) and 8-15 parts propylene glycol diacetate (PGDA) as a high-boiling-point film-forming aid and a carrier for lactide, based on 100 parts by weight of the aqueous polyurethane emulsion; PGDA is used as a solvent for lactide, ensuring complete dissolution of the lactide to form a premix before adding it to the emulsion. This prevents the solid lactide from directly contacting large amounts of water and undergoing hydrolysis, thus ensuring storage stability.

[0012] Component C: Additives added during coating formulation, comprising 3-6 parts by weight of waterborne polyurethane emulsion: epoxy-grafted modified nano-silica as a reinforcing filler to improve hardness and abrasion resistance; 1-5 parts by weight of amino-functionalized light stabilizer microcapsules for resisting photoaging and chemically anchoring to the network; 0.1-0.2 parts by weight of leveling agent (BYK-346) to improve leveling properties; 0.1-0.5 parts by weight of defoamer (BYK-024) to eliminate bubbles; and an appropriate amount of polyurethane thickener to adjust the viscosity to 2000-5000 cP.

[0013] The above formula design is based on the following ideas: 1. Molecular design of latent blocked isocyanates The blocking agent used in this invention is N,N-dimethylethanolamine (DMEA), whose general molecular formula is: HO-CH2-CH2-N(CH3)2; This molecule has a dual function: Hydroxyl group (-OH): Used to block isocyanate groups (-NCO) to form urethane closure bonds; Tertiary amine group (-N(CH3)2): As the initiating active center, it has catalytic activity to initiate the ring-opening polymerization of lactide after decongestion.

[0014] DMEA, as a tertiary amine, possesses catalytic activity and can catalyze the reaction of isocyanate groups with water, polyols, and other substances. The initial unblocking temperature of DMEA-blocked isocyanates is 130°C. DMEA's dual role as both a blocking agent and an organic catalyst / initiator for the ring-opening polymerization of lactide is the core design basis of this invention.

[0015] Literature studies have shown that aliphatic tertiary amines (such as triethylamine (TEA) and N,N,N',N'-tetramethylethylenediamine (TMEDA)) can catalyze the ring-opening polymerization of lactide. DMEA, as a tertiary amine containing a hydroxyl group, has its hydroxyl group acting as an initiating active center, while the tertiary amine acts as a catalyst; the synergistic effect of both can achieve highly efficient ring-opening polymerization of lactide.

[0016] 2. Closed reaction The hydroxyl groups in the blocking agent molecule react with the isocyanate groups at the ends of the isocyanate prepolymer to form urethane blocking bonds: OCN-PU-NCO+2HO-CH2-CH2-N(CH3)2→ (CH3)2N-CH2-CH2-O-CO-NH-PU-NH-CO-O-CH2-CH2-N(CH3)2; In the blocking reaction, the hydroxyl group (-OH) of DMEA undergoes a nucleophilic addition reaction with the -NCO group to form a carbamate bond (-NH-CO-O-), protecting the -NCO group. This blocking bond is stable at room temperature, giving the emulsion excellent storage stability.

[0017] 3. Unsealing-polymerization linkage reaction During the thermosetting stage (130-150℃), the closed urethane bonds of DMEA break, releasing active NCO and dissociating into free DMEA: (CH3)2N-CH2-CH2-O-CO-NH-PU→OCN-PU+HO-CH2-CH2-N(CH3)2 (heating); The released NCO reacts with the residual hydroxyl groups on the polyurethane chains in a cross-linking reaction: OCN-PU+HO-PU'→PU-NH-COO-PU'; Simultaneously, the tertiary amine group (-N(CH3)2) in the dissociated DMEA molecule exhibits catalytic activity under thermal action, immediately initiating the ring-opening polymerization of the premixed lactide monomer in the system. The mechanism of tertiary amine-catalyzed ring-opening polymerization of lactide is as follows: the tertiary amine acts as a nucleophile, attacking the carbonyl carbon of lactide to form a tetrahedral intermediate, followed by acyl-oxygen bond cleavage, leading to ring-opening of the lactide, while the tertiary amine departs and is regenerated. The hydroxyl group in the DMEA molecule can act as an initiating active center, and the tertiary amine acts as a catalyst, synergistically initiating the ring-opening polymerization of lactide. The polymerization reaction can be represented as: HO-CH2-CH2-N(CH3)2+n lactide → HO-CH2-CH2-N(CH3)2-O-(PLA) n -H (heating); In the formula, the hydroxyl group of DMEA acts as the initiating active center, and the tertiary amine acts as a catalyst, synergistically initiating the ring-opening of lactide, with the polylactic acid segment growing from the end of DMEA. The suitable temperature range for the ring-opening polymerization of lactide is 100-180℃, and 130-150℃ is the commonly used temperature range for bulk melt polymerization.

[0018] 4. Synergistic Effects and Chemical Connections Cross-linking network formation: NCO reacts with hydroxyl groups to construct a three-dimensional cross-linking framework, which imparts hardness, media resistance and mechanical strength to the coating.

[0019] In-situ formation of polylactic acid: PLA segments generated by the ring-opening polymerization of lactide are uniformly dispersed in the cross-linked network at the nanoscale, playing an internal toughening role, reducing the internal stress of the coating, and improving flexibility and adhesion.

[0020] Chemical Linkage: The hydroxyl group of the DMEA molecule serves as both the blocking group of NCO and the initiation point for lactide ring-opening. After unblocking, DMEA remains chemically anchored to the polyurethane chain through its hydroxyl residue (-O-) at the site of the previous urethane bond breakage; simultaneously, its tertiary amine group initiates lactide polymerization, and PLA segments grow from the ends of DMEA. Therefore, the PLA segments form a covalent chemical link with the polyurethane network through DMEA residues (-NH-CO-O-CH2-CH2-N(CH3)2-O-(PLA)-), which is an in-situ chemical grafting, not a physical blending.

[0021] No byproduct residue: The blocking agent DMEA no longer remains in the coating as a free small molecule, but is transformed into the end groups of PLA segments and chemically anchored to the network, eliminating the problem of traditional blocking agent residue.

[0022] Based on the above analysis, the present invention also proposes a preparation process for the aforementioned modified polyurethane waterborne functional coating, comprising the following steps: Step 1: Synthesis of Latent Blocked Isocyanate-Ended Waterborne Polyurethane Emulsion (1) Raw material dehydration: Add PCDL to a four-necked flask and dehydrate under vacuum at 110-120℃ for 1 hour until the moisture content is <0.05%.

[0023] (2) Prepolymerization reaction: Cool down to 75℃, add IPDI and catalyst DBTDL (0.05-0.1 parts), and heat up to 80℃ under nitrogen protection, react for 2-3 hours. Take samples during the reaction and determine the -NCO content by di-n-butylamine titration until the NCO content reaches the theoretical value.

[0024] Prepolymerization reaction equation: OCN-R-NCO+HO-PCDL-OH→OCN-R-NH-COO-PCDL-OCO-NH-R-NCO; (3) Hydrophilic chain extension: Cool to 70℃, add DMPA and BDO, and continue the reaction for 1.5-2h. DMPA introduces carboxyl groups (-COOH) as hydrophilic groups, providing hydrophilicity for subsequent water dispersion.

[0025] Chain extension reaction equation: OCN-prepolymer-NCO+HO-DMPA-OH+HO-BDO-OH→chain extended prepolymer (containing carboxyl groups). (4) Blocking isocyanate end-capping: Cool down to 50-55℃, slowly add DMEA blocking agent (6-15 parts), keep the reaction at the temperature for 1-1.5h until -NCO is completely consumed, and confirm that the blocking reaction is complete.

[0026] Closed reaction equation: OCN-prepolymer-NCO+2HO-CH2-CH2-N(CH3)2→ (CH3)2N-CH2-CH2-O-CO-NH-prepolymer-NH-CO-O-CH2-CH2-N(CH3)2; (5) Neutralization: Cool to 38-40℃ and add TEA (4-7 parts) to neutralize for 30 min. TEA reacts with the carboxyl groups of DMPA to form ammonium carboxylate salt, which enhances the water dispersibility of the polymer.

[0027] Neutralization reaction: -COOH + N(C2H5)3 → -COO - +HN(C2H5)3 + ; (6) Emulsification: Under high-speed stirring (2000-3000 rpm), slowly add deionized water (180-250 parts) and emulsify for 30-40 minutes to obtain a semi-transparent blue light emulsion. The solid content is about 32-36%.

[0028] Step 2: Preparation of lactide-film-forming aid premix L-lactide (5-20 parts) was completely dissolved in propylene glycol diacetate (PGDA, 8-15 parts) under stirring at 60°C to obtain a clear and transparent lactide-PGDA premix. This premix remained stable at room temperature (lactide was dissolved or supersaturated in PGDA but did not precipitate) and was ready for use.

[0029] Design Principle: Lactide readily hydrolyzes and opens its ring to form lactic acid upon contact with water, posing a core technical challenge in introducing lactide into aqueous systems. This invention employs a non-aqueous premixing method, pre-dissolving lactide in a hydrophobic film-forming aid PGDA to form a uniform dispersion before mixing it with an aqueous polyurethane emulsion. PGDA encapsulates the lactide microdroplets, isolating them from the aqueous phase and fundamentally inhibiting the hydrolysis reaction. Simultaneously, PGDA, as a high-boiling-point film-forming aid (approximately 191°C), slowly volatilizes during the infrared preheating stage (80-90°C), allowing lactide to precipitate uniformly between polyurethane microparticles, distributing as nanocrystals within the coating film. This provides a uniform monomer dispersion for the subsequent hot-air baking stage of ring-opening polymerization.

[0030] Step 3: Preparation of amino-functionalized light stabilizer microcapsules (1) Microcapsule core material and wall material: Microcapsules were prepared by in-situ polymerization using a light stabilizer (such as UV-328 or Tinuvin 292) as the core material and melamine-formaldehyde resin as the wall material. Specifically, the light stabilizer was dissolved in toluene as the oil phase, and the aqueous solution of melamine and formaldehyde was used as the aqueous phase. The mixture was emulsified at high speed in the presence of an emulsifier to form an O / W emulsion. The mixture was heated to 60-70℃ and reacted for 2-3 hours. Melamine and formaldehyde condensed on the surface of the oil droplets to form the wall material, thus obtaining a microcapsule suspension.

[0031] (2) Surface amino functionalization: After the microcapsule preparation is completed, γ-aminopropyltriethoxysilane (KH-550, 2-5% of the mass of the microcapsules) is added, and the mixture is stirred at 50-60℃ for 1 h. After hydrolysis, KH-550 reacts with the hydroxyl groups on the surface of the microcapsules to introduce primary amino groups (-NH2) onto the surface of the microcapsules, thus obtaining amino-functionalized microcapsules. After centrifugation, washing, and drying, the microcapsules are ready for use.

[0032] Functionalization principle: During the thermosetting stage, the primary amines on the surface of the microcapsules react with the NCO released after desealing to form substituted urea bonds (-NH-CO-NH-), which covalently anchor the microcapsules to the cross-linked network, eliminating physical interface defects and ensuring the sustained-release stability of the light stabilizer during long-term service.

[0033] Step 4: Preparation of the coating Take 100 parts of the aqueous polyurethane emulsion prepared in step one, and add it sequentially while stirring at 400 rpm: (1) The lactide-PGDA premix prepared in step two (containing 5-20 parts lactide and 8-15 parts PGDA). (2) Epoxy-grafted modified nano-SiO2 (3-6 parts); (3) The amino-functionalized light stabilizer microcapsules prepared in step three (1-5 parts); (4) Leveling agent (0.1-0.2 parts); Heat to 800 rpm and disperse at high speed for 15 minutes to ensure uniform dispersion of all components.

[0034] Reduce the speed to 300 rpm, add defoamer (0.1-0.5 parts) and polyurethane thickener (appropriate amount) in sequence, and adjust the viscosity to 2000-5000 cP (25℃, Brookfield DV-II viscometer, No. 3 rotor, 12 rpm).

[0035] After filtration through a 100-mesh stainless steel screen, the modified polyurethane water-based functional coating is obtained.

[0036] Step 5: Coating and Curing Apply the coating to the substrate surface (by spraying, scraping, or rolling), and control the wet film thickness to 0.2-0.5 mm.

[0037] Two-stage curing process: The first stage—infrared preheating for dehydration: Infrared preheating at 80-90℃ for 10-15 minutes. During this stage, moisture evaporates, and the emulsion particles fuse into a continuous coating film; PGDA film-forming aid partially evaporates, and lactide precipitates uniformly, distributing as nanocrystals within the polyurethane matrix. Lactide is a solid (melting point approximately 95-98℃) and has not yet melted or polymerized at 80-90℃.

[0038] The second stage—hot air baking and unsealing-polymerization linkage: hot air circulation baking at 130-150℃ for 20-30 minutes. This stage triggers a linkage reaction: DMEA-blocked isocyanate is unsealed (initial unsealing temperature approximately 130℃), releasing NCO to crosslink with the hydroxyl groups on the polyurethane chain; the unsealed DMEA tertiary amine groups are activated under heat, initiating the ring-opening polymerization of lactide monomers (130-150℃ is the commonly used temperature range for bulk melt polymerization of lactide); lactide is completely melted at this temperature, and polymerization proceeds efficiently in the molten state. The epoxy groups on the surface of modified nano-SiO2 simultaneously participate in the ring-opening reaction, anchoring to the crosslinking network. The -NH2 on the surface of the amino-functionalized microcapsules reacts with NCO to form urea bonds, chemically anchoring them to the coating.

[0039] The modified polyurethane waterborne functional coating is obtained by naturally cooling to room temperature.

[0040] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention uses N,N-dimethylethanolamine (DMEA) as an isocyanate blocking agent, combined with the premixing and introduction of lactide monomer and a two-stage curing process. Through the deblocking-polymerization chemical linkage mechanism, it solves the technical problem of coating performance degradation caused by inert byproducts remaining after the blocking agent is deblocked in traditional blocked isocyanate systems. At the same time, it overcomes the contradiction between the crosslinking density and flexibility of waterborne polyurethane crosslinked coatings.

[0041] 2. In this invention, the DMEA molecule contains both hydroxyl and tertiary amine groups. The hydroxyl group is used to block isocyanate from forming a latent crosslinking agent, while the tertiary amine group acts as an active initiation center for the ring-opening polymerization of lactide after thermal desealing. This molecular design upgrades the single-use protection function of traditional blocking agents to a dual protection-initiation function, transforming the blocking agent from an inert byproduct discarded upon desealing into an active species that initiates the process upon desealing. This eliminates the coating performance degradation problem caused by blocking agent residue at the molecular design level.

[0042] 3. This invention pre-dissolves lactide monomer in propylene glycol diacetate (PGDA) film-forming aid to form a premix before adding it to an aqueous polyurethane emulsion. Utilizing the hydrophobicity and lactide-dissolving ability of PGDA, a physical isolation and protection is formed for lactide in an aqueous environment, effectively inhibiting the hydrolysis and ring-opening reaction of lactide in the aqueous phase. This solves the technical problem of poor long-term storage stability of lactide in aqueous emulsions. After the PGDA slowly volatilizes during the infrared preheating film-forming stage, lactide is uniformly precipitated and distributed in the polyurethane matrix in the form of nanocrystals, providing a uniform monomer dispersion for subsequent thermally initiated polymerization.

[0043] 4. This invention employs a two-stage curing process. After infrared preheating for dehydration and film formation, the unblocking crosslinking of DMEA-blocked isocyanate and the ring-opening polymerization of lactide are simultaneously triggered during the 130-150℃ hot air baking stage. The released NCO reacts with the hydroxyl groups on the polyurethane chains to construct a three-dimensional crosslinked network, endowing the coating with high hardness and excellent resistance to media. Simultaneously, the dissociated tertiary amine groups of DMEA immediately initiate the ring-opening polymerization of lactide. The generated polylactic acid segments form covalent chemical connections with the polyurethane network through DMEA residues, achieving in-situ chemical toughening by uniformly dispersing at the nanoscale within the crosslinked network. The chemical linkage between the crosslinking and polymerization reactions enables the coating to maintain high hardness while achieving excellent flexibility and impact resistance, thus unifying high crosslinking density and high flexibility.

[0044] 5. This invention uses amino-functionalized light stabilizer microcapsules, whose surface primary amino groups react with the NCO released during the thermosetting stage to generate substituted urea bonds, chemically anchoring the microcapsules to the cross-linked network with covalent bonds. This eliminates the interface defects of traditional physically blended microcapsules, ensures the sustained-release stability of the light stabilizer during long-term service, and significantly improves the weather resistance and durability of the coating.

[0045] 6. The lactide monomer selected in this invention is derived from renewable biomass resources (lactic acid). The PLA segments generated by in-situ polymerization have good biodegradability and biocompatibility. DMEA and PGDA are both low-toxicity and environmentally friendly additives. The entire coating system has low VOC content, which is in line with the direction of green and sustainable development. Detailed Implementation

[0046] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0047] I. Synthesis of Latent Blocked Isocyanate-Ended Waterborne Polyurethane Emulsion: Synthesis Example 1 (Preferred Scheme): Formula: IPDI 32 parts, PCDL 45 parts, DMPA 6 parts, BDO 3 parts, DMEA 10 parts, TEA 5 parts, deionized water 200 parts, DBTDL 0.06 parts.

[0048] Process: (1) Add PCDL to a four-necked flask and dehydrate under vacuum at 110°C for 1 hour until the moisture content is <0.05%; (2) Cool down to 75°C, add IPDI and DBTDL, heat up to 80°C under nitrogen protection, react for 2.5 h, and take a sample to titrate the NCO content to reach the theoretical value (theoretical NCO=6.8%). (3) Cool down to 70°C, add DMPA (dissolved in a small amount of NMP) and BDO, and react for 1.5 h; (4) Cool to 50℃, slowly add DMEA, maintain the temperature for 1 hour, and monitor the NCO peak (2270 cm⁻¹) using FTIR. - ¹) disappear completely; (5) Cool down to 38°C, add TEA and neutralize for 30 minutes; (6) Add deionized water slowly under high-speed stirring at 2000 rpm, emulsify for 30 min, remove a small amount of NMP by vacuum distillation, and obtain a semi-transparent blue light emulsion with a solid content of about 34.2%.

[0049] Synthesis Example 2 (lower limit of DMEA dosage): The dosage of DMEA was 6 parts, and the rest was the same as in Synthesis Example 1. An emulsion with a solid content of approximately 33.8% was obtained.

[0050] Synthesis Example 3 (maximum DMEA dosage): The dosage of DMEA was 15 parts, and the rest was the same as in Synthesis Example 1. An emulsion with a solid content of approximately 34.5% was obtained.

[0051] Comparative Synthesis Example 1 (Traditional Butanol Blocking Agent – ​​No Initiation Function): DMEA was replaced with an equimolar amount of n-butanol (n-BuOH) for blocking, with the rest remaining the same as Synthesis Example 1. After unblocking, n-butanol lacks tertiary amine groups and therefore does not possess the ability to initiate lactide polymerization.

[0052] Comparative Synthesis Example 2 (without lactide monomer): The synthesis is the same as in Synthesis Example 1, but lactide monomer is not added during coating formulation.

[0053] Comparative Synthesis Example 3 (microcapsules without amination): The microcapsules were not amination treated with KH-550, and the rest were the same as in Synthesis Example 1.

[0054] Comparative Synthesis Example 4 (No Heat Curing - Room Temperature Drying): The coating formulation is the same as that of Synthesis Example 1, but the curing process only involves 7 days of room temperature drying after infrared preheating at 80°C, without hot air baking.

[0055] Comparative Synthesis Example 5 (insufficient curing temperature - baking at 100℃): The coating formulation is the same as that of Synthesis Example 1, and the hot air baking temperature is 100℃ / 25min (lower than the DMEA-blocked isocyanate unblocking initiation temperature of 130℃ and the lactide melting point of 95-98℃).

[0056] Comparative Synthesis Example 6 (physical blending with added PLA): In Synthesis Example 1, no lactide monomer was added. When preparing the coating, an equal amount of polylactic acid (PLA, Mn=5000) was added for physical blending. The rest was the same as in Synthesis Example 1.

[0057] Comparative Synthesis Example 7 (lactide directly added to emulsion – no PGDA pre-dissolution): In Synthesis Example 1, lactide was directly added to the aqueous polyurethane emulsion in solid powder form without pre-dissolving in PGDA, and the rest was the same as in Synthesis Example 1.

[0058] Comparative Synthesis Example 8 (curing temperature too high - baking at 170℃): The coating formulation is the same as Synthesis Example 1, and the hot air baking temperature is 170℃ / 20min.

[0059] II. Coating Preparation and Curing Based on the above synthesis example, and supplemented with the raw materials listed in Table 1 below: Table 1. Raw Materials and Reagents Isophorone diisocyanate (IPDI) Industrial grade, ≥99% Wanhua Chemical Polycarbonate diol (PCDL) Mn=2000, hydroxyl value 56mg KOH / g Ube Industries Dimethylolpropionic acid (DMPA) Industrial grade, ≥98% Perima 1,4-Butanediol (BDO) Industrial grade, ≥99% BASF N,N-Dimethylethanolamine (DMEA) Industrial grade, ≥99% Yangba Triethylamine (TEA) Industrial grade, ≥99% BASF Dibutyltin dilaurate (DBTDL) Industrial grade, ≥95% Air Chemicals L-lactide Industrial grade, ≥99% Jindan Technology Propylene glycol diacetate (PGDA) Industrial grade, ≥99% Eastman <![CDATA[Epoxy group grafted modified nano-SiO2]]> Particle size 20-40nm, epoxy value 0.3-0.5mmol / g Evonik KH-550 Industrial grade, ≥98% Nanjing Shuguang BYK-346 Industrial grade BYK Chemical BYK-024 Industrial grade BYK Chemical UV-328 light stabilizer Industrial grade, ≥99% BASF melamine Industrial grade, ≥99% Sichuanhua Formaldehyde (37% aqueous solution) Industrial grade Sinopharm The coatings and processes of the following embodiments and comparative examples are shown in Table 2 below: Table 2. Coating formulation schemes for each embodiment and comparative example: Example 1 Synthesis example 1 10 12 10 Amination 80℃ / 12min + 140℃ / 25min Example 2 Synthesis example 2 6 8 8 Amination 80℃ / 12min + 140℃ / 25min Example 3 Synthesis example 3 15 18 14 Amination 80℃ / 12min + 140℃ / 25min Example 4 Synthesis example 1 10 5 8 Amination 80℃ / 12min + 140℃ / 25min Example 5 Synthesis example 1 10 20 15 Amination 80℃ / 12min + 140℃ / 25min Example 6 Synthesis example 1 10 12 10 Amination 80℃ / 12min + 130℃ / 30min Example 7 Synthesis example 1 10 12 10 Amination 80℃ / 12min + 150℃ / 20min Comparative Example 1 Comparative Synthesis Example 1 0 (Butanol) 12 10 Amination 80℃ / 12min + 140℃ / 25min Comparative Example 2 Comparative Synthesis Example 2 10 0 0 Amination 80℃ / 12min + 140℃ / 25min Comparative Example 3 Comparative Synthesis Example 3 10 12 10 Unaminated 80℃ / 12min + 140℃ / 25min Comparative Example 4 Comparative Synthesis Example 4 10 12 10 Amination 80℃ / 12min + room temperature for 7 days Comparative Example 5 Comparative Synthesis Example 5 10 12 10 Amination 80℃ / 12min + 100℃ / 25min Comparative Example 6 Comparative Synthesis Example 6 10 0 (+PLA blend) 0 Amination 80℃ / 12min + 140℃ / 25min Comparative Example 7 Comparative Synthesis Example 7 10 12 0 Amination 80℃ / 12min + 140℃ / 25min Comparative Example 8 Comparative Synthesis Example 8 10 12 10 Amination 80℃ / 12min + 170℃ / 20min Uniform preparation procedures for all embodiments and comparative examples: Take 100 parts of the corresponding emulsion, add lactide (or lactide-PGDA premix, according to the ratio in the table), 3 parts of aminated microcapsules (except for Comparative Example 3), 5 parts of modified nano-SiO2, and 0.15 parts of leveling agent, disperse at 800 rpm for 15 min; reduce the speed to 300 rpm, add 0.3 parts of defoamer, adjust the viscosity to 3000±200 cP with thickener, and filter through 100 mesh.

[0060] Curing process (except for comparative examples 4, 5, and 8): Tinplate (50mm×120mm×0.3mm) is sanded, degreased, cleaned, and dried for later use. The coating is sprayed onto the tinplate surface to a wet film thickness of 0.3mm; preheated at 80℃ for 12 minutes using infrared technology; then transferred to a hot air oven and baked according to the conditions in the table; allowed to cool naturally to room temperature and left to stand for 24 hours before testing.

[0061] Comparative Example 4: After infrared preheating at 80℃ for 12 minutes, it was placed at room temperature for 7 days.

[0062] Comparative Example 5: After infrared preheating at 80℃ for 12 minutes, it was baked with hot air at 100℃ for 25 minutes.

[0063] Comparative Example 8: After infrared preheating at 80℃ for 12 minutes, it was baked with hot air at 170℃ for 20 minutes.

[0064] Table 3. Performance Testing Methods Cross-grip adhesion GB / T 9286-2021 Levels 1-5, Level 1 is the best Pencil hardness GB / T 6739-2006 Mitsubishi pencil, scratch method Impact resistance GB / T 1732-2020 Forward strike, kg·cm flexibility GB / T 1731-2020 Shaft bending method, mm Cupping test GB / T 9753-2007 mm Gloss (60°) GB / T 9754-2007 gloss meter Water resistance GB / T 1733-1993 Method A Soak in water for 240 hours and observe for bubbling / peeling / discoloration. Salt spray resistance GB / T 1771-2007 500h, 5% NaCl, 35℃ Resistance to damp heat aging GB / T 1740-2007 47℃ / 96%RH / 1000h QUV aging resistance GB / T 23987-2009 UVA-340, 1000h, light retention rate measured. Abrasion resistance (Taber) GB / T 1768-2006 CS-10 grinding wheel, 1000g / 500r, mg Storage stability 50℃ / 60d Observe sedimentation and measure viscosity change rate. gel content Acetone extraction method Characterizing crosslink density Method for determining gel content: Weigh the cured coating (approximately 0.5 g) (W1), place it in a Soxhlet extractor, and reflux extract with acetone as solvent for 24 h. Remove the residue, vacuum dry at 60 °C to constant weight, and weigh it (W2). Gel content = W2 / W1 × 100%.

[0065] The performance test results are as follows: Table 4. Physical properties of coatings Example 1 1 3H 55 1 9.5 94 91.2 Example 2 1 2H 50 1 9.2 92 87.5 Example 3 1 3H 55 1 9 93 93.8 Example 4 1 3H 48 2 8.5 93 92 Example 5 1 3H 58 1 9.8 92 90.5 Example 6 1 3H 52 1 9.3 93 88.6 Example 7 1 3H 55 1 9.5 94 92.8 Comparative Example 1 2 2H 35 2 7.2 87 78.3 Comparative Example 2 1 3H 30 3 6.5 91 93.5 Comparative Example 3 1 3H 50 1 9 92 90.8 Comparative Example 4 3 B 50 1 10.5 85 12.6 Comparative Example 5 2 H 40 2 7.5 88 65.4 Comparative Example 6 2 2H 38 2 7 86 92.8 Comparative Example 7 2 2H 45 2 8 89 85.2 Comparative Example 8 2 4H 25 3 5.5 90 95.2 Table 5. Coating resistance to media and durability Example 1 No bubbling, no peeling, no discoloration No blistering, no rust, adhesion level 1 18 No bubbling, adhesion grade 1, ΔE=1.3 94 Example 2 No bubbling, slight loss of gloss Small amount of microbubbles, adhesion level 1 25 Small amount of microbubbles, adhesion grade 1, ΔE=2.0 90 Example 3 No bubbling, no peeling, no discoloration No blistering, no rust, adhesion level 1 16 No bubbling, adhesion grade 1, ΔE=1.1 96 Example 4 No bubbling, no peeling Small amount of microbubbles, adhesion level 1 22 Slight bubbling, adhesion grade 1, ΔE=2.5 91 Example 5 No bubbling, no peeling, no discoloration No blistering, no rust, adhesion level 1 15 No bubbling, adhesion grade 1, ΔE=0.9 95 Example 6 No bubbling, no peeling, no discoloration Small amount of microbubbles, adhesion level 1 20 Slight bubbling, adhesion grade 1, ΔE=1.8 92 Example 7 No bubbling, no peeling, no discoloration No blistering, no rust, adhesion level 1 17 No bubbling, adhesion grade 1, ΔE=1.2 95 Comparative Example 1 Blistering, localized softening Localized blistering, corrosion, adhesion level 3 42 Localized blistering, adhesion grade 3, ΔE=5.8 68 Comparative Example 2 No bubbling, no peeling Small amount of microbubbles, adhesion level 1 30 Slight bubbling, adhesion level 2, ΔE=3.8 84 Comparative Example 3 No bubbling, no peeling No bubbling, adhesion level 1 20 Slight bubbling, adhesion level 2, ΔE=5.5 70 Comparative Example 4 Large-area bubbling, softening, and peeling Coating failure 80 Coating failure Coating failure Comparative Example 5 Blistering, localized softening Large-area blistering and corrosion 58 Large-area bubbling, adhesion level 4 38 Comparative Example 6 Blistering, localized peeling Localized blistering, corrosion, adhesion level 3 45 Extensive bubbling, adhesion level 4, ΔE=7.5 60 Comparative Example 7 Slight bubbling, slight loss of shine Localized blistering, adhesion level 2 35 Bubbling, adhesion grade 3, ΔE=4.2 78 Comparative Example 8 No bubbling, slight yellowing No blistering, no rust, adhesion level 1 14 No bubbling, adhesion grade 1, ΔE=3.5 85 Table 6. Storage Stability Example 1 3050 3150 3.3 No settlement No (PGDA Package Protection) Example 2 2980 3080 3.4 No settlement no Example 3 3120 3220 3.2 No settlement no Example 4 3000 3100 3.3 No settlement no Example 5 3080 3200 3.9 No settlement no Example 6 3050 3150 3.3 No settlement no Example 7 3050 3150 3.3 No settlement no Comparative Example 6 3100 3380 9 PLA partially swells and aggregates. — Comparative Example 7 3080 3650 18.5 Extensive sedimentation and stratification Yes (lactide hydrolysis) The following conclusions can be drawn from Table 4-6: 1. Unblocking-Aggregation Linkage Mechanism: Comparison of Example 1 with Comparative Examples 1 and 2 Comparative Example 1 used a traditional butanol sealant, which lacks tertiary amine initiation activity. After the butanol-blocked isocyanate was deblocked at 140°C, the butanol remained in the coating as a free small molecule. Since no PLA was generated, the coating formed a network solely through the cross-linking reaction of NCO and hydroxyl groups. Test results showed: impact resistance of only 35 kg·cm, flexibility of 2 mm, cupping of 7.2 mm, adhesion grade 2, and gel content of 78.3% (lower than 91.2% in Example 1). The butanol residue as a small molecule after deblocking not only acted as a plasticizer, degrading coating performance, but also caused microporous defects due to partial butanol volatilization. Salt spray resistance showed corrosion, adhesion dropped to grade 3, and QUV gloss retention was only 68%. This indicates that although the traditional sealant system achieved cross-linking, the sealant residue severely limited the overall performance of the coating.

[0066] Comparative Example 2 did not add lactide monomer, but used DMEA as a blocking agent. Although no lactide could initiate polymerization after DMEA deblocking, its tertiary amine groups remained in the coating. The impact resistance was only 30 kg·cm, the flexibility was 3 mm, and the cupping was 6.5 mm—typical brittleness of a pure crosslinked system. The gel content was as high as 93.5%, indicating a very high crosslinking density. However, the brittleness brought about by the high crosslinking density resulted in severely insufficient flexibility and impact resistance of the coating. This verifies a core argument of this invention: simply increasing the crosslinking density cannot simultaneously achieve high hardness and high flexibility; an internal toughening phase must be introduced into the crosslinking network.

[0067] Example 1 contains both DMEA blocking agent and lactide monomer. Under curing conditions of 140°C (the initial unblocking temperature of DMEA-blocked isocyanate is 130°C), the DMEA-blocked isocyanate unblocks and releases NCO to complete crosslinking (gel content 91.2%). Simultaneously, the dissociated DMEA tertiary amine groups initiate the ring-opening polymerization of lactide to generate PLA segments (130-150°C is the commonly used temperature range for bulk melt polymerization of lactide). The PLA segments are uniformly chemically grafted onto the crosslinked network at the nanoscale, serving as an internal toughening phase. This allows the coating to maintain a high hardness of 3H while achieving an impact resistance of 55 kg·cm, a flexibility of 1 mm, and a cupping of 9.5 mm. Water resistance, salt spray resistance, damp heat aging resistance, and QUV resistance all reach optimal levels.

[0068] Traditional sealants are discarded upon release, but this invention initiates the polymerization reaction upon release. DMEA is upgraded from an inert protecting group to a polymerization trigger, simultaneously completing crosslinking curing and polymer toughening. The PLA segment has a covalent chemical connection point with the polyurethane network through DMEA residues (-NH-CO-O-CH2-CH2-N(CH3)2-O-(PLA)-), which is an in-situ chemical grafting, not a physical blending, fundamentally eliminating the contradiction between sealant residue and the high brittleness of the crosslinked coating.

[0069] 2. DMEA Dosage Optimization: A Horizontal Comparison of Examples 1-5 Example 2 (6 parts DMEA, lower limit): DMEA serves as both a blocking agent and an initiator / catalyst precursor. When used in smaller quantities, the number of NCOs that can be blocked is limited, resulting in a relatively low crosslinking density (gel content 87.5%). Simultaneously, less free DMEA is generated after deblocking, leading to a relatively low lactide conversion rate, low PLA formation, and a mild toughening effect. Hardness 2H, flexibility 1mm, impact resistance 50kg·cm—all properties are within the effective range but not optimal.

[0070] Example 3 (DMEA 15 parts, maximum): Sufficient DMEA resulted in a high number of blocked NCOs and a high crosslinking density (gel content 93.8%). After deblocking, a large amount of free DMEA was generated, leading to a high lactide conversion rate, sufficient PLA production, and the strongest toughening effect. Hardness remained at 3H, impact resistance was 55 kg·cm, flexibility was 1 mm, and abrasion resistance was optimal (16 mg). However, excessive DMEA dosage may have resulted in some DMEA not fully participating in initiation (due to limited lactide content), and residual trace amounts of tertiary amines may have a slight impact on long-term water resistance.

[0071] Example 4 (5 parts lactide, lower limit): The amount of PLA produced was relatively small, resulting in limited toughening effect. The impact resistance was 48 kg·cm, and the flexibility was 2 mm. Although the hardness remained at 3H, the flexibility was slightly inferior to that of Example 1.

[0072] Example 5 (20 parts of lactide, upper limit): The PLA production was sufficient, resulting in the strongest toughening effect, with an impact resistance of 58 kg·cm, a flexibility of 1 mm, and a cupping of 9.8 mm. However, excessive lactide may cause some PLA segments to be too long, slightly affecting the uniformity and gloss of the coating (92 vs. 94 in Example 1).

[0073] Example 1 (10 parts DMEA, 12 parts lactide): The ratio of initiator to lactide is balanced, the crosslinking density and PLA generation reach the best balance, and the overall performance is optimal.

[0074] 3. Precise control of curing temperature: Comparison of Example 1 with Comparative Examples 4, 5, and 8 Comparative Example 4 (Room Temperature Drying): After only 80°C infrared preheating followed by room temperature drying, the unblocking temperature of DMEA-blocked isocyanate (approximately 130°C) was far from being reached, and lactide did not polymerize. The gel content was only 12.6%, indicating that the coating is essentially a linear thermoplastic structure. Adhesion was grade 3, hardness was B, and water resistance and salt spray resistance were completely ineffective. This proves that hot air baking is a necessary condition for triggering the linkage.

[0075] Comparative Example 5 (baking at 100°C): 100°C is below the initial unblocking temperature of DMEA-blocked isocyanate (130°C), triggering only a very small amount of DMEA unblocking. Meanwhile, lactide has a melting point of approximately 95-98°C; at 100°C, lactide just melts, but lacks a sufficient amount of initiator, resulting in a very incomplete polymerization reaction. The gel content is 65.4%, far lower than the 91.2% of Example 1. Adhesion is grade 2, hardness is H, and impact resistance is 40 kg·cm—performance significantly inferior to Example 1. This demonstrates that 130°C is the minimum effective temperature for the linkage between unblocking and polymerization.

[0076] Comparative Example 8 (baked at 170℃): 170℃ is much higher than the temperature required for unsealing, and the unsealing and cross-linking reactions proceed more thoroughly, with a gel content as high as 95.2% and a hardness increase to 4H. However, the excessively high temperature leads to: (1) the cross-linking reaction being too intense, the network structure being too rigid, the impact resistance dropping sharply to 25 kg·cm, the flexibility being 3 mm, and the cupping being only 5.5 mm; (2) high temperature may cause thermal degradation or excessive cross-linking of PLA, affecting the toughening effect; (3) the coating showing slight yellowing (ΔE=3.5 vs 1.3 of Example 1). This indicates that higher temperatures are not necessarily better, and 140℃ is the optimal temperature for achieving a balance between cross-linking and toughening.

[0077] Examples 1, 6, and 7 were cured at 130℃ / 30min, 140℃ / 25min, and 150℃ / 20min, respectively. All three groups exhibited excellent overall performance, with 140℃ / 25min (Example 1) showing the best overall performance. At 130℃ (Example 6), the baking time needed to be extended to 30min to ensure sufficient reaction; at 150℃ (Example 7), the reaction could be completed within 20min, but the high temperature placed higher demands on energy consumption and substrate adaptability.

[0078] 4. Advantages of in-situ polymerization compared to physical blending: Comparison of Example 1 and Comparative Example 6 Comparative Example 6 used an equal amount of PLA (Mn=5000) in physical blending to replace in-situ polymerization of lactide. Impact resistance was 38 kg·cm, flexibility was 2 mm, cupping was 7.0 mm, and adhesion was grade 2, all significantly weaker than Example 1. Storage stability was poor: PLA partially swelled and agglomerated in the emulsion, with a viscosity change rate of +9.0% after 50°C / 60 days (compared to only +3.3% in Example 1). QUV gloss retention was only 60%.

[0079] Physically blended PLA exists as an independent phase, interacting with the polyurethane matrix only through weak van der Waals forces, making it prone to phase separation and migration. PLA segments generated by in-situ polymerization of lactide grow from the ends of the DMEA initiator, forming covalent chemical bonds with the crosslinking network through DMEA residues, achieving uniform dispersion at the molecular scale. The in-situ toughening efficiency of chemical bonding is far superior to that of physical blending—this is the key feature that distinguishes this invention from existing technologies.

[0080] 5. Contribution of PGDA premix to storage stability: Comparison of Example 1 and Comparative Example 7 In Comparative Example 7, lactide was added directly to an aqueous polyurethane emulsion in solid powder form without prior dissolution in PGDA. After accelerated storage at 50°C for 60 days, the viscosity increased significantly from 3080 cP to 3650 cP (change rate +18.5%), and a large amount of precipitation and stratification occurred; lactic acid may have appeared in the sample, i.e., lactide underwent hydrolysis and ring-opening in the aqueous phase to generate lactic acid.

[0081] Example 1 employed a PGDA pre-dissolution method, where lactide was encapsulated by PGDA to form microdroplets, effectively inhibiting direct contact with the aqueous phase and suppressing hydrolysis. After storage at 50°C for 60 days, the viscosity increased only from 3050 cP to 3150 cP (change rate +3.3%), with no sedimentation or stratification, and no lactic acid characteristic peak was detected by FTIR.

[0082] Lactide is sensitive to water; when directly dispersed in an aqueous system, water molecules can penetrate to the lactide surface, initiating nucleophilic hydrolysis and ring-opening. PGDA, as a hydrophobic, high-boiling-point film-forming aid, encapsulates lactide to form a protective layer, providing physical isolation in the aqueous environment. Simultaneously, PGDA volatilizes during the film-forming stage, allowing lactide to precipitate in situ within the coating film without affecting subsequent thermally initiated polymerization. The PGDA premix strategy is a key supporting technology of this invention, fundamentally solving the hydrolytic stability problem of lactide during long-term storage in aqueous emulsions.

[0083] 6. Contribution of microencapsulation amino functionalization: Comparison of Example 1 and Comparative Example 3 Comparative Example 3 microcapsules were not subjected to KH-550 amination treatment. Their conventional physical properties (adhesion, hardness, impact resistance, flexibility, cupping) were similar to those of Example 1, which is as expected—the microcapsules primarily function as a slow-release carrier for the light stabilizer, with limited direct impact on the mechanical properties of the coating.

[0084] However, there were significant differences in long-term durability: Comparative Example 3 showed blistering and ΔE=5.5 in resistance to damp heat aging (1.3 in Example 1), and the QUV gloss retention rate was only 70% (94% in Example 1).

[0085] Unaminated microcapsules are physically embedded in the coating, with the microcapsule wall material and the polyurethane matrix relying solely on physical interlocking and weak interfacial forces. During humid heat aging or QUV aging, microcracks may form at the interface due to thermal expansion and contraction or moisture penetration, leading to premature release and failure of the light stabilizer. Simultaneously, interfacial debonding creates light scattering centers, accelerating gloss degradation and color changes. In contrast, the primary amino groups (-NH2) on the surface of amino-functionalized microcapsules react with the released NCO during the thermosetting stage to form substituted urea bonds (-NH-CO-NH-), chemically anchoring the microcapsules to the cross-linked network via covalent bonds. This chemical anchoring eliminates physical interfacial defects, ensuring the sustained-release stability of the light stabilizer during long-term service.

[0086] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A modified polyurethane waterborne functional coating, characterized in that, The product is composed of a latent isocyanate-terminated aqueous polyurethane emulsion, lactide monomer, and functional additives. The latent isocyanate isocyanate-terminated by N,N-dimethylethanolamine, in which the hydroxyl group is used to block the isocyanate group, and the tertiary amine group has the activity of initiating the ring-opening polymerization of lactide after deblocking. The lactide monomer is pre-dissolved in propylene glycol diacetate film-forming aid to form a premix before being added to the aqueous polyurethane emulsion.

2. The modified polyurethane waterborne functional coating according to claim 1, characterized in that, The latent isocyanate-terminated aqueous polyurethane emulsion is made from the following raw materials in parts by weight: 28-38 parts isophorone diisocyanate, 40-55 parts polycarbonate diol, 5-8 parts dimethylolpropionic acid, 2-5 parts 1,4-butanediol, 6-15 parts N,N-dimethylethanolamine, 4-7 parts triethylamine, 180-250 parts deionized water, and 0.05-0.1 parts catalyst.

3. The modified polyurethane waterborne functional coating according to claim 1, characterized in that, The lactide monomer is L-lactide, D-lactide, or DL-lactide. The amount of lactide monomer used is 5-20 parts per 100 parts by weight of the aqueous polyurethane emulsion; the amount of propylene glycol diacetate used is 8-15 parts per 100 parts by weight of the aqueous polyurethane emulsion.

4. The modified polyurethane waterborne functional coating according to claim 1, characterized in that, The functional additives, based on 100 parts by weight of the waterborne polyurethane emulsion, include: 3-6 parts of epoxy-grafted modified nano-silica, 1-5 parts of amino-functionalized light stabilizer microcapsules, 0.1-0.2 parts of leveling agent, 0.1-0.5 parts of defoamer, and an appropriate amount of polyurethane thickener.

5. The modified polyurethane waterborne functional coating according to claim 4, characterized in that, The amino-functionalized light stabilizer microcapsules use melamine-formaldehyde resin as the wall material and light stabilizer as the core material. The surface is treated with γ-aminopropyltriethoxysilane to graft primary amino groups, which react with the isocyanate groups released during the thermosetting stage to form urea bonds, chemically anchored to the cross-linked network.

6. The preparation process of the modified polyurethane waterborne functional coating according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Dehydrate polycarbonate diol under vacuum at 110-120℃ until the moisture content is <0.05%; (2) Cool down to 75°C, add isophorone diisocyanate and catalyst, and react at 80°C for 2-3 hours under nitrogen protection until the NCO content reaches the theoretical value; (3) Cool down to 70°C, add dimethylolpropionic acid and 1,4-butanediol and react for 1.5-2 hours; (4) Cool to 50-55℃, add N,N-dimethylethanolamine and maintain the temperature for 1-1.5h. (5) Cool down to 38-40℃ and add triethylamine to neutralize for 30 minutes; (6) Add deionized water under high-speed stirring to emulsify and obtain a latent blocked isocyanate-terminated waterborne polyurethane emulsion; (7) Dissolve the lactide monomer in propylene glycol diacetate under stirring at 60°C to obtain a lactide premix; (8) Take 100 parts of the emulsion from step (6), add the premixed liquid and functional additives from step (7), disperse evenly, adjust the viscosity to 2000-5000 cP, and filter out the material.

7. The preparation process of the modified polyurethane waterborne functional coating according to claim 6, characterized in that, In step (8), the dispersion conditions are as follows: after adding each component in sequence under stirring at 400 rpm, disperse at high speed at 800 rpm for 15 min, and then reduce to 300 rpm to add defoamer and thickener.

8. A curing method for a modified polyurethane waterborne functional coating according to any one of claims 1-5, characterized in that, A two-stage curing process is used: First stage: Infrared preheating at 80-90℃ for 10-15 minutes to remove moisture and form a continuous coating film; The second stage involves hot air baking at 130-150℃ for 20-30 minutes, which triggers the deblocking of N,N-dimethylethanolamine-blocked isocyanate to release active NCO and crosslink with the hydroxyl groups on the polyurethane chain. At the same time, the dissociated tertiary amine groups of N,N-dimethylethanolamine initiate the ring-opening polymerization of lactide monomer, achieving simultaneous crosslinking curing and polymer toughening.

9. The curing method according to claim 8, characterized in that, The second stage involves baking with hot air at 140℃ for 25 minutes.

10. The coating prepared by the curing method according to claim 8, characterized in that, In the coating, polylactic acid segments are covalently chemically linked to the polyurethane crosslinking network through N,N-dimethylethanolamine residues, achieving in-situ chemical toughening.