A high and low temperature resistant environmentally friendly water-based polyurethane coating and its preparation method

Abstract

The present invention discloses a high- and low-temperature resistant environmentally friendly water-based polyurethane coating and a preparation method thereof, belonging to the technical field of chemical coatings. The preparation method comprises: reacting isophorone diisocyanate with cyanuric acid at 80-90°C to generate a prepolymer containing a triazine ring cross-linked network; adding hyperbranched polycaprolactone to the prepolymer, and vacuum dehydrating the prepolymer to form a mixed prepolymer; using dimethylolpropionic acid to extend the chain and neutralizing it with triethylamine, adding a coating additive and shearing and emulsifying to obtain a high- and low-temperature resistant environmentally friendly water-based polyurethane coating. Through the synergistic effect of the rigid triazine ring network and the hyperbranched chain segments, the coating has both brittle crack resistance and creep resistance over a wide temperature range; the amphiphilic chain extender ensures emulsion stability and low VOC content.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical coatings, and in particular relates to a high and low temperature resistant and environmentally friendly water-based polyurethane coating and a preparation method thereof. Background Art

[0002] Waterborne polyurethane coatings are widely used in the automotive, electronics, and construction industries due to their environmental friendliness (low VOC emissions) and excellent mechanical properties. Traditional waterborne polyurethane coatings typically utilize linear polyurethane chains as their base structure, with water dispersibility achieved through the introduction of hydrophilic groups. However, the heat resistance of these coatings is limited by the trade-off between molecular chain flexibility and crosslink density, resulting in a narrow long-term operating temperature range and difficulty meeting the demands of applications in extreme environments.

[0003] Conventional waterborne polyurethane coatings generally have low-temperature resistance above -30°C. At low temperatures, molecular chain motion is hindered, making the coating susceptible to brittle fracture. At high temperatures (above 120°C), molecular chain thermal motion intensifies, causing the crosslinked network to loosen, leading to coating softening and even adhesion. While some studies have attempted to improve temperature resistance by adding silicone modifications or inorganic fillers, these often result in increased VOC content or a significant decrease in coating flexibility, making it difficult to balance environmental performance with wide-temperature stability.

[0004] Patent CN105802431A discloses a waterborne polyurethane coating modified by grafting amino and isocyanate groups onto the surface of diatomaceous earth, then compounding it with hydroxyl-terminated polybutadiene and acrylic resin. This solution uses inorganic fillers such as montmorillonite and kaolin to enhance mechanical properties, while leveraging the adsorption properties of diatomaceous earth to impart formaldehyde purification capabilities to the coating. However, this approach has the following limitations: First, the use of silane coupling agents and toluene diisocyanate may increase VOC content, conflicting with environmental protection requirements. Second, while the introduction of large amounts of inorganic fillers improves high-temperature resistance, it also exacerbates low-temperature embrittlement, and multiphase interfacial compatibility issues can easily lead to reduced coating uniformity. Furthermore, the complex multi-step graft polymerization process (including diatomaceous earth pretreatment, prepolymer synthesis, and acrylate copolymerization) significantly increases production energy consumption and complicates quality control, making efficient industrial production difficult. Therefore, balancing temperature resistance, flexibility, and environmental performance within a wider temperature range (-60°C to 160°C) while streamlining the preparation process remains a technical bottleneck in this field.

[0005] Furthermore, existing technologies often use excessive amounts of hydrophilic chain extenders to improve emulsion stability, which degrades the coating's water resistance. High-temperature curing processes (such as single-stage high-temperature treatment) can easily lead to increased microphase separation, affecting coating uniformity. Therefore, developing a waterborne polyurethane coating that combines excellent mechanical strength, flexibility, and low VOC properties across a wide temperature range of -60°C to 160°C remains a pressing technical challenge in this field. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the present invention provides a method for preparing a high- and low-temperature resistant environmentally friendly water-based polyurethane coating, comprising the following steps: S1: mixing isophorone diisocyanate with cyanuric acid, stirring and reacting at 80-90°C at 250-350 rpm for 2.5-3.5 hours under nitrogen protection to obtain a prepolymer containing a triazine ring cross-linked network; S2: adding hyperbranched polycaprolactone to the prepolymer, wherein the mass ratio of the hyperbranched polycaprolactone to the isophorone diisocyanate is 0.45:1, and stirring at 60-70°C at 200-300 rpm for 1 hour. Stirring the mixture for reaction, and vacuum dehydrating for 2.5-3.5 hours to obtain a mixed prepolymer; S3: adding dimethylolpropionic acid chain extender to the mixed prepolymer, adding triethylamine for neutralization after the reaction, adjusting the pH to 8.5-9.0, then adding deionized water for emulsification and dispersion at a shear rate of 4500-5500 rpm, and adding a coating additive to obtain a high and low temperature resistant environmentally friendly water-based polyurethane coating; wherein the mass ratio of the mixed prepolymer, the dimethylolpropionic acid chain extender and the coating additive is 100:(4-5):(1.3-4.3).

[0007] In some embodiments, the molar ratio of isophorone diisocyanate to cyanuric acid in S1 is 2.8:1 to 3.2:1.

[0008] In some embodiments, the reaction temperature of S1 is 83-87° C., the reaction time is 2.8-3.2 h, and the stirring rate is 280-320 rpm.

[0009] In some embodiments, the molecular weight of the hyperbranched polycaprolactone in S2 is 3000-4000, and the vacuum degree during vacuum dehydration is -0.090 to -0.098 MPa.

[0010] In some embodiments, the molar ratio of triethylamine to dimethylolpropionic acid is 1.05-1.15:1.

[0011] In some embodiments, the emulsification dispersion time of S3 is 25-35 min, and the emulsification temperature is 55-65°C.

[0012] In some embodiments, the hyperbranched polycaprolactone is prepared by the following steps: ring-opening polymerization of caprolactone and pentaerythritol at a molar ratio of hydroxyl group to ester group of 1:8-12, adding stannous octoate as a catalyst at a concentration of 0.5-1.2% by weight of caprolactone, and reacting at 120-140° C. under nitrogen protection for 4-6 hours to obtain a hyperbranched polycaprolactone having a branching degree of 15-25% and a hydroxyl value of 45-55 mg KOH / g.

[0013] The present invention also provides a high and low temperature resistant and environmentally friendly water-based polyurethane coating prepared by any of the aforementioned methods.

[0014] In some embodiments, the gradient temperature curing includes: first curing at 75-85°C for 50-70 minutes, and then curing at 115-125°C for 25-35 minutes. The aging treatment includes: curing for 20-28 hours in an environment with a humidity of 40-60%.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] 1. By introducing the rigid cross-linked network of triazine rings of cyanuric acid, the prepolymer forms a high cross-linking density structure, which effectively inhibits the thermal motion of molecular segments at high temperatures and gives the coating excellent high-temperature softening resistance after curing.

[0017] 2. By grafting hyperbranched polycaprolactone flexible chain segments, dynamic flexible micro-regions are formed in the molecular chain, which inhibits brittle cracking caused by chain segment crystallization at low temperatures (-40°C) and improves the low-temperature impact resistance of the coating after curing.

[0018] 3. Through the gradient temperature curing and aging process, the coating constructs a rigid-flexible interpenetrating network in stages, reducing thermal stress concentration and enhancing dimensional stability in a wide temperature range.

[0019] 4. Through the use of amphiphilic chain extenders and high shear emulsification technology, the polyurethane segments form stable micellar dispersions, which improves the storage stability of the aqueous emulsion and increases the density of the coating after curing.

[0020] 5. Through the design of a fully water-based system and the selection of a low-volatility neutralizer, the VOC content of the coating is reduced, and no harmful gases are released during the curing process.

[0021] 6. By synergistically controlling the molecular weight of hyperbranched polycaprolactone and the degree of dehydration vacuum, the compatibility of the soft and hard segments is optimized, the interfacial bonding strength is enhanced, and cracking of the coating after curing caused by phase separation is avoided.

[0022] 7. By controlling the curing temperature and aging humidity in stages, the cross-linking network is deeply improved, and the coating's resistance to cold and hot cycles is significantly enhanced after curing.

[0023] 8. By precisely controlling the amount of dimethylolpropionic acid added and the neutralization molar ratio, the hydrophilic-hydrophobic balance and water resistance are optimally matched. DETAILED DESCRIPTION

[0024] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0025] The present invention provides a method for preparing a high and low temperature resistant environmentally friendly water-based polyurethane coating, comprising the following steps:

[0026] S1: mixing isophorone diisocyanate and cyanuric acid, stirring at 250-350 rpm and 80-90°C for 2.5-3.5 hours under nitrogen protection to obtain a prepolymer containing a triazine ring cross-linked network;

[0027] S2: adding hyperbranched polycaprolactone to the prepolymer, wherein the mass ratio of the hyperbranched polycaprolactone to the isophorone diisocyanate is 0.45:1, stirring at 200-300 rpm at 60-70° C., and simultaneously dehydrating under vacuum for 2.5-3.5 hours to obtain a mixed prepolymer;

[0028] S3: adding dimethylolpropionic acid chain extender to the mixed prepolymer, adding triethylamine for neutralization after the reaction, adjusting the pH to 8.5-9.0, then adding deionized water at a shear rate of 4500-5500 rpm for emulsification and dispersion, and adding coating additives to obtain a high and low temperature resistant and environmentally friendly water-based polyurethane coating;

[0029] Wherein, the mass ratio of the mixed prepolymer, the dimethylol propionic acid chain extender and the coating additive is 100:(4-5):(1.3-4.3).

[0030] In the S1 stage, the isocyanate groups (-NCO) of isophorone diisocyanate (IPDI) and the hydroxyl groups (-OH) of cyanuric acid undergo a polycondensation reaction, connecting them through urethane bonds (-NHCOO-) to form a prepolymer. The rigid triazine ring structure of cyanuric acid is introduced into the polyurethane backbone, forming a three-dimensional crosslinked network. A reaction temperature of 80-90°C under nitrogen protection accelerates molecular chain motion while preventing side reactions caused by high temperatures. A stirring rate of 250-350 rpm promotes uniform mixing of the monomers, and a reaction time of 2.5-3.5 hours allows for balanced crosslinking density, imparting high-temperature softening resistance to the cured coating.

[0031] In stages S2 and S3, the branched structure of hyperbranched polycaprolactone (HB-PCL) is grafted onto residual -NCO groups in the prepolymer via hydroxyl groups. A 0.45:1 mass ratio of HB-PCL to isophorone diisocyanate creates flexible microdomains within the molecular chain, inhibiting brittle cracking caused by crystallization at low temperatures. Because isocyanate groups are highly sensitive to water, even trace amounts of water can react. Vacuum dehydration (-0.095 MPa) at 60-70°C removes any traces of water to prevent reaction with -NCO groups and uncontrolled crosslinking. Dimethylolpropionic acid (DMBA) is subsequently added to extend the chain by reacting with the -NCO groups in the prepolymer. Its carboxylic acid groups (-COOH) are neutralized with triethylamine to form carboxylate salts (-COO⁻), imparting amphiphilic properties to the polyurethane segments. High shear emulsification at 4500-5500 rpm allows the hydrophobic segments to encapsulate the hydrophilic groups, forming stable micelles. A pH of 8.5-9.0 ensures sufficient ionization of the carboxylate salts, enabling aqueous dispersion.

[0032] By introducing the synergistic effect of the triazine ring cross-linking network and hyperbranched polycaprolactone, the coating has both brittle crack resistance and softening resistance in a wide temperature range after curing; the design of the amphiphilic chain extender improves the stability of the water-based emulsion. At the same time, through the selection of a fully water-based system and a low-volatility neutralizer, the VOC content of the coating is significantly reduced, meeting environmental protection requirements.

[0033] In some embodiments, the coating additives include a defoamer, a leveling agent, and a film-forming aid in a mass ratio of 1:(2-3):(5-8).

[0034] In some embodiments, the defoaming agent is polydimethylsiloxane, the leveling agent is polyether-modified polysiloxane, and the film-forming aid is 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate.

[0035] In some embodiments, the hyperbranched polycaprolactone is prepared by the following steps:

[0036] Caprolactone and pentaerythritol are subjected to a ring-opening polymerization reaction at a molar ratio of hydroxyl group to ester group of 1:8-12, and stannous octoate (0.5-1.2% by weight of caprolactone) is added as a catalyst. The reaction is carried out at 120-140° C. under nitrogen protection for 4-6 hours to obtain a hyperbranched polycaprolactone with a branching degree of 15-25% and a hydroxyl value of 45-55 mg KOH / g.

[0037] In some embodiments, the molar ratio of isophorone diisocyanate to cyanuric acid in S1 is 2.8:1 to 3.2:1.

[0038] When the molar ratio is lower than 3:1 but higher than 2.8:1, a relative excess of cyanuric acid can increase the triazine ring crosslink density, but sufficient isocyanate groups must be retained for subsequent chain extension. When the molar ratio is higher than 3:1 but lower than 3.2:1, excess isocyanate groups are used to ensure chain extension activity. This ratio dynamically balances the rigidity of the crosslinked network with the efficiency of the chain extension reaction, allowing the prepolymer to form a rigid skeleton that resists high-temperature softening while retaining sufficient active groups to react with subsequent hyperbranched polycaprolactone and chain extenders. This prevents the deterioration of the coating's mechanical properties after curing due to excessive crosslinking or insufficient chain extension, while also improving the tolerance of the raw material feeding process.

[0039] In some embodiments, the reaction temperature of S1 is 83-87° C., the reaction time is 2.8-3.2 h, and the stirring rate is 280-320 rpm.

[0040] By limiting the reaction temperature to 83-87°C, the reaction time to 2.8-3.2 hours, and the stirring rate to 280-320 rpm, a balance is established between the kinetics and thermodynamics of the polycondensation reaction. Temperatures below 83°C result in an incomplete crosslinking network due to insufficient reaction rate, while temperatures above 87°C accelerate the self-polymerization of the isocyanate groups. A reaction time of 2.8-3.2 hours ensures sufficient reaction of the cyanuric acid hydroxyl groups, preventing prepolymer gelation caused by excessive crosslinking. A stirring rate of 280-320 rpm ensures uniform monomer dispersion while preventing high-speed shear from damaging the triazine ring crosslinking structure. By controlling the reaction process, the molecular weight distribution of the prepolymer is narrowed, minimizing uneven segment entanglement during the subsequent chain extension stage and improving the high-temperature creep resistance of the cured coating.

[0041] In some embodiments, the molecular weight of the hyperbranched polycaprolactone in S2 is 3000-4000, and the vacuum degree during vacuum dehydration is -0.090 to -0.098 MPa.

[0042] Molecular weights below 3000 result in short branched chains, leading to insufficient flexibility. Molecular weights above 4000 reduce interfacial bonding with the triazine ring network due to excessively long chain segments. Low vacuum levels (>-0.090 MPa) during vacuum dehydration can lead to residual moisture, triggering subsequent chain extension side reactions, while high vacuum levels (<-0.098 MPa) accelerate the volatilization of low-boiling-point monomers, disrupting the formulation. By synergistically controlling molecular weight matching and the degree of dehydration, the soft segments are evenly interspersed within the rigid network, suppressing low-temperature brittle cracking while maintaining high-temperature dimensional stability. By limiting the molecular weight of hyperbranched polycaprolactone to 3000-4000 and the vacuum dehydration degree to -0.090 to -0.098 MPa, the compatibility of the soft and hard segments is optimized during the S2 stage.

[0043] In some embodiments, the molar ratio of triethylamine to dimethylolpropionic acid is 1.05-1.15:1.

[0044] When the content of dimethylolpropionic acid is less than 4.2%, insufficient carboxylic acid groups lead to poor emulsion dispersion stability. When it is higher than 4.8%, excessive hydrophilic segments reduce the water resistance of the coating. A triethylamine molar ratio of 1.05-1.15:1 ensures that the carboxylic acid groups are completely neutralized to form carboxylates (avoiding unneutralized acid from causing emulsion flocculation) without excessive residual free amines (leading to increased VOC). By precisely controlling the hydrophilic-hydrophobic balance, the emulsion particle size is uniform and a continuous and dense structure is formed after the coating is cured, taking into account both environmental protection and mechanical strength.

[0045] In some embodiments, the emulsification dispersion time of S3 is 25-35 min, and the emulsification temperature is 55-65°C.

[0046] By limiting the emulsification and dispersion time of S3 to 25-35 minutes and the emulsification temperature to 55-65°C, the micelle formation kinetics and thermal stability are balanced during the shear emulsification process. When the emulsification time is less than 25 minutes, the high shear force cannot fully break up the polyurethane segments to form uniform micelles, while exceeding 35 minutes may destroy the already formed stable micelle structure. Since emulsification becomes difficult when the temperature is too low, the increased segment rigidity makes emulsification difficult, while too high a temperature triggers self-polymerization of unreacted isocyanate groups. The temperature range of 55-65°C maintains moderate mobility of the polyurethane segments. By matching the shear time and temperature window, the emulsion particle size distribution is concentrated, avoiding the rough coating surface caused by excessively large particles or the deterioration of permeability caused by too small particles, thereby improving the uniformity and adhesion of the coating after curing.

[0047] The present invention also provides a high and low temperature resistant and environmentally friendly water-based polyurethane coating prepared by any of the aforementioned methods.

[0048] The present invention also provides a method for preparing a high and low temperature resistant environmentally friendly water-based polyurethane coating, comprising: applying the high and low temperature resistant environmentally friendly water-based polyurethane coating, and then sequentially performing surface drying at room temperature, gradient temperature curing and aging treatment to obtain the high and low temperature resistant environmentally friendly water-based polyurethane coating.

[0049] After emulsion coating, a gradient curing strategy is used to build the coating structure in stages: room temperature surface drying allows for slow evaporation of water, preventing cracking. A preliminary curing step at 80°C for 1 hour promotes secondary crosslinking of -NCO groups with residual hydroxyl groups and water, forming an initial network. A high-temperature treatment at 120°C for 30 minutes accelerates further crosslinking of unreacted groups, eliminating thermal stress. Finally, a 24-hour aging step rearranges and densifies the molecular chains. The synergistic effect of the triazine ring crosslinking network and the HB-PCL flexible chains maintains the coating's mechanical stability over a wide temperature range. This gradient curing process results in a dense interpenetrating network after curing.

[0050] In some embodiments, the gradient temperature curing includes: first curing at 75-85°C for 50-70 minutes, and then curing at 115-125°C for 25-35 minutes. The aging treatment includes: aging for 20-28 hours in an environment with a humidity of 40-60%. In the first stage, 75-85°C allows the residual isocyanate groups to slowly react with the hydroxyl groups to generate secondary cross-linking points, forming a preliminary network skeleton; in the second stage, 115-125°C activates deep unreacted groups, promotes the interpenetration of the triazine ring cross-linking network and the hyperbranched polycaprolactone chain segments, and reduces the internal defects of the coating after curing by releasing thermal stress in stages, while avoiding the aggravation of microphase separation caused by single high-temperature curing, thereby enhancing the coating's resistance to thermal creep and cold and hot cycling in a wide temperature range.

[0051] When the humidity is lower than 40%, the environment is too dry, which hinders the movement of molecular chains and causes insufficient curing, resulting in internal stress concentration in the coating. When the humidity is higher than 60%, water penetration triggers premature hydrolysis of unreacted groups, destroying the integrity of the cross-linked network. A curing time of 20-28 hours ensures that the molecular chains are fully oriented through Brownian motion. Through the synergistic effect of humidity and time, the rigid network of triazine rings and the hyperbranched flexible chain segments form an interlocking structure, thereby improving the coating interface bonding strength and fatigue resistance.

[0052] The method of the present invention will be described in detail below with reference to embodiments, comparative examples and experimental data.

[0053] Example 1

[0054] This embodiment provides a method for preparing a high and low temperature resistant environmentally friendly water-based polyurethane coating, comprising the following steps:

[0055] S1: 300 g of isophorone diisocyanate and 42.6 g of cyanuric acid were added to a reactor, nitrogen was introduced, the temperature was raised to 85° C., and the mixture was stirred at 300 rpm for 2.5 h to obtain a prepolymer containing a triazine ring cross-linked network.

[0056] S2: Weigh 135 g of hyperbranched polycaprolactone and add it to the prepolymer obtained in S1. Maintain the system temperature at 65°C and stir at 250 rpm for 45 minutes. Then dehydrate at a vacuum degree of -0.095 MPa for 3 hours to remove water and low-boiling substances to obtain a mixed prepolymer.

[0057] S3: Cool the mixed prepolymer to 60°C, add 13.5g of dimethylol propionic acid, react for 1h, add 10.2g of triethylamine, and adjust the pH to 8.8. Subsequently, deionized water is added at a shear rate of 5000rpm and an emulsification temperature of 60°C to a solid content of 58%. At the same time, 1.875g of polydimethylsiloxane is added as a defoaming agent, 3.75g of polyether-modified polysiloxane is added as a leveling agent, and 9.375g of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate is added as a film-forming aid. Emulsify and disperse for 30min to obtain a high and low temperature resistant and environmentally friendly water-based polyurethane coating.

[0058] Among them, hyperbranched polycaprolactone is prepared by the following method: caprolactone and pentaerythritol are added to a reactor at a molar ratio of hydroxyl group to ester group of 1:10, and then stannous octoate (0.85% by weight of caprolactone) is added as a catalyst. After nitrogen is introduced to replace the air, the temperature is raised to 130°C and the reaction is continued with stirring for 5 hours to obtain hyperbranched polycaprolactone with an average molecular weight of 3000-4000.

[0059] Example 2

[0060] This embodiment provides a method for preparing a high- and low-temperature resistant environmentally friendly water-based polyurethane coating, comprising the following steps:

[0061] S1: 300 g of isophorone diisocyanate and 40.0 g of cyanuric acid were added to a reactor, nitrogen was introduced, the temperature was raised to 90° C., and the mixture was stirred at 350 rpm for 3.0 h to obtain a prepolymer containing a triazine ring cross-linked network.

[0062] S2: 135 g of hyperbranched polycaprolactone was weighed and added to the prepolymer obtained in S1. The system temperature was maintained at 60°C and the reaction was stirred at 200 rpm for 30 min. The mixture was then dehydrated at a vacuum degree of -0.098 MPa for 3.5 h to remove water and low-boiling substances to obtain a mixed prepolymer.

[0063] S3: Cool the mixed prepolymer to 60°C, add 13.8g of dimethylolpropionic acid, react for 1h, add 10.5g of triethylamine, and adjust the pH to 8.8. Subsequently, deionized water is added at a shear rate of 5000rpm and an emulsification temperature of 65°C to a solid content of 58%. At the same time, 1.875g of polydimethylsiloxane is added as a defoaming agent, 3.75g of polyether-modified polysiloxane is added as a leveling agent, and 9.375g of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate is added as a film-forming aid. Emulsify and disperse for 35min to obtain a high and low temperature resistant and environmentally friendly water-based polyurethane coating.

[0064] Among them, hyperbranched polycaprolactone is prepared by the following method: caprolactone and pentaerythritol are added to a reactor at a molar ratio of hydroxyl group to ester group of 1:12, and then stannous octoate (0.5% by weight of caprolactone) is added as a catalyst, nitrogen is introduced into the reaction mixture to replace the air, and the temperature is raised to 140°C. The reaction is stirred and continued for 6 hours to obtain hyperbranched polycaprolactone with an average molecular weight of 3000-4000.

[0065] Example 3

[0066] This embodiment provides a method for preparing a high- and low-temperature resistant environmentally friendly water-based polyurethane coating, comprising the following steps:

[0067] S1: 300 g of isophorone diisocyanate and 45.5 g of cyanuric acid were added to a reactor, nitrogen was introduced, the temperature was raised to 80° C., and the mixture was stirred at 250 rpm for 3.5 h to obtain a prepolymer containing a triazine ring cross-linked network.

[0068] S2: 135 g of hyperbranched polycaprolactone was weighed and added to the prepolymer obtained in S1. The system temperature was maintained at 60°C and the reaction was stirred at 300 rpm for 60 min. The mixture was then dehydrated at a vacuum degree of -0.090 MPa for 2.5 h to remove water and low-boiling substances to obtain a mixed prepolymer.

[0069] S3: Cool the mixed prepolymer to 60°C, add 12.6g of dimethylolpropionic acid, react for 1h, add 9.8g of triethylamine, and adjust the pH to 8.8. Subsequently, deionized water is added at a shear rate of 5000rpm and an emulsification temperature of 55°C to a solid content of 58%. At the same time, 1.875g of polydimethylsiloxane is added as a defoaming agent, 3.75g of polyether-modified polysiloxane is added as a leveling agent, and 9.375g of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate is added as a film-forming aid. Emulsify and disperse for 25min to obtain a high and low temperature resistant and environmentally friendly water-based polyurethane coating.

[0070] Among them, hyperbranched polycaprolactone is prepared by the following method: caprolactone and pentaerythritol are added to a reactor at a molar ratio of hydroxyl group to ester group of 1:8, and then stannous octoate (1.2% by weight of caprolactone) is added as a catalyst. After nitrogen is introduced to replace the air, the temperature is raised to 120°C and the reaction is continued with stirring for 4 hours to obtain hyperbranched polycaprolactone with an average molecular weight of 3000-4000.

[0071] Comparative Example 1

[0072] This comparative example provides a method for preparing a high- and low-temperature resistant environmentally friendly water-based polyurethane coating, which differs from Example 1 in that: in S1, the amount of isophorone diisocyanate is 300 g, and the amount of cyanuric acid is 64.5 g.

[0073] Comparative Example 2

[0074] This comparative example provides a method for preparing a high- and low-temperature resistant environmentally friendly water-based polyurethane coating. The difference from Example 1 is that no cyanuric acid is added in S1, and only isophorone diisocyanate and hyperbranched polycaprolactone are reacted in S2.

[0075] Comparative Example 3

[0076] This comparative example provides a method for preparing a high and low temperature resistant environmentally friendly water-based polyurethane coating. The difference from Example 1 is that hyperbranched polycaprolactone is not added to S2, but is replaced by an equal mass of linear polycaprolactone (Mn=3500).

[0077] Comparative Example 4

[0078] This comparative example provides a method for preparing a high and low temperature resistant and environmentally friendly water-based polyurethane coating. The difference from Example 1 is that the amount of dimethylolpropionic acid added in S3 is 3.5% of the total mass of the prepolymer.

[0079] Comparative Example 5

[0080] This comparative example provides a preparation method of a high- and low-temperature resistant environmentally friendly water-based polyurethane coating. The difference from Example 1 is that the emulsification and dispersion time in S3 is 15 minutes and the emulsification temperature is 45°C.

[0081] Comparative Example 6

[0082] This comparative example provides a method for preparing a high- and low-temperature resistant environmentally friendly water-based polyurethane coating. The difference from Example 1 is that the vacuum degree of vacuum dehydration in S2 is -0.080 MPa, and the dehydration time is 1 hour.

[0083] The performance of the high and low temperature resistant and environmentally friendly water-based polyurethane coating provided in the above embodiments and comparative examples was tested. First, a high and low temperature resistant and environmentally friendly water-based polyurethane coating was prepared according to the following method:

[0084] The high and low temperature resistant environmentally friendly water-based polyurethane coating was applied to the surface of the substrate. After drying at room temperature for 30 minutes, it was first cured at 80°C for 1 hour, then heated to 120°C for 30 minutes, and finally matured in a humidity environment of 50% for 24 hours to obtain a 50μm thick coating.

[0085] The coating is then tested using the following method:

[0086] 1. High-temperature softening temperature: The coating sample (10×10×1 mm³) was heated from 25°C to 200°C at a heating rate of 5°C / min using a thermomechanical analyzer (TMA, TA Instruments Q400). The temperature corresponding to 1% strain on the deformation curve was recorded.

[0087] 2. Low-temperature crack resistance: Place the coating sample (100×25×0.05 mm³) in a high-low temperature test chamber (ESPECSU-221) and cool it down to the target temperature (-70°C~25°C) at a rate of 10°C / min. After holding the temperature for 30 minutes, perform a 180° bending test and observe the lowest temperature at which surface cracks are generated.

[0088] 3. VOC content: Determined in accordance with GB 38508-2020 using a gas chromatography-mass spectrometer (Agilent 7890B / 5977B). A 1g emulsion sample was heated at 120°C for 1 hour to capture volatile organic compounds (VOCs). Total VOC content was quantitatively calculated using the internal standard method (toluene as the internal standard).

[0089] 4. Water absorption: Immerse the coating sample (50 × 50 × 0.05 mm³) in deionized water at 25°C. After 240 hours, remove the sample, dry the surface moisture, and measure the mass change: Water absorption (%) = (mass after immersion - initial mass) / initial mass × 100%.

[0090] 5. Adhesion (cross-hatch method): In accordance with ISO 2409, use a six-blade cutting knife (blade spacing 2mm) to create a 6×6 grid on the coating surface. Apply 3M 610 tape and quickly peel it off at a 60° angle. The adhesive grade is assessed based on the percentage of the area removed (Grade 0: no removal; Grade 1: ≤5%; Grade 2: 5-15%).

[0091] 6. Hot and cold cycle test: The coating sample was cycled between -60℃ (30min) and 160℃ (30min) 10 times, with each switching time less than 5min. After the cycle, the surface cracks and peeling were observed using an optical microscope (Olympus BX53).

[0092] 7. Surface roughness Ra: Use a white light interferometer (Bruker ContourGT-K) to scan the coating surface (5×5 mm² area) and take the arithmetic average roughness Ra value (measurement resolution 0.1 nm).

[0093] 8. Internal stress: The substrate bending method was used to measure the change in substrate curvature radius before and after curing using a laser displacement sensor (Keyence LK-G5000). The internal stress was calculated using the following formula:

[0094] σ=Es·ts² / (6(1-vs)·tf)·(1 / Rf-1 / Ri)

[0095] Where Es = 160 GPa (silicon wafer elastic modulus), vs = 0.22 (Poisson's ratio), ts = 0.5 mm, tf = 50 μm, Ri represents the initial curvature radius of the substrate before coating, and Rf represents the final curvature radius of the substrate after coating is cured.

[0096] The above performance test data is shown in Table 1.

[0097] Table 1 Performance test results

[0098]

[0099] Experimental results analysis

[0100] In Example 1, the coating exhibited no cracking or softening within the temperature range of -60°C to 160°C, a VOC content of 42 g / L, a water absorption rate of 2.8%, adhesion level 0, a surface roughness of Ra = 0.15 μm, and an internal stress of 8 MPa. Its wide temperature range stability is attributed to the synergistic effect of the triazine ring cross-linked network and hyperbranched polycaprolactone. Gradient curing and sufficient maturation result in a dense, defect-free coating.

[0101] In Example 2, the high-temperature softening temperature slightly dropped to 158°C, the low-temperature crack resistance was -58°C, the VOC was 45g / L, and the water absorption rate was 3.1%, indicating that the performance could still be maintained under process limit conditions. However, the surface roughness (Ra = 0.18μm) increased slightly, which may be related to the slight gelation of the prepolymer at higher temperatures.

[0102] In Example 3, the low-temperature crack resistance temperature is increased to -62°C, but the high-temperature softening temperature is increased to 162°C, and the water absorption rate is 2.5%, indicating that lower crosslinking density and more complete dehydration can optimize low-temperature resistance, but high-temperature stability needs to be balanced.

[0103] In Comparative Example 1, the high-temperature softening temperature is only 135°C, it cracks at -40°C, and the VOC is 50g / L, indicating that insufficient cross-linking network density leads to deterioration of temperature resistance, and the increase of unreacted isocyanate groups triggers side reactions (increased VOC).

[0104] In Comparative Example 2, the high-temperature softening temperature is 110°C, the adhesion is level 2, and severe peeling occurs after hot and cold cycles, proving that the loss of the triazine ring causes the coating to lose its high-temperature creep resistance and significantly reduces the interfacial bonding strength.

[0105] In Comparative Example 3, the low-temperature crack resistance temperature is only -40°C, the high-temperature modulus decreases by 50%, and the surface roughness Ra = 0.25 μm, indicating that the linear segment cannot inhibit low-temperature crystallization and has poor compatibility with the rigid network.

[0106] In Comparative Example 4, VOC increased to 68 g / L, the water absorption rate was 7.5%, and emulsion stratification resulted in fish-eye defects in the coating film, confirming that the chain extender was insufficient and solvent needed to be added, and that too few hydrophilic groups affected the stability of the emulsion.

[0107] In Comparative Example 5, the surface roughness Ra is 1.2 μm, the water absorption rate is 8%, and the emulsion particle size D90 is greater than 500 nm, indicating that insufficient emulsification leads to a loose micelle structure and deteriorated permeability of the coating film.

[0108] In Comparative Example 6, the water content > 0.5% resulted in a ±30% fluctuation in the emulsion viscosity, and the coating film bubbled after being immersed in water for 48 hours, verifying the effect of incomplete dehydration on water resistance.

[0109] Examples 1-3 achieve high-stability coatings over a wide temperature range (-60°C to 160°C) by utilizing a triazine ring cross-linked network (high-temperature resistance ≥158°C), hyperbranched polycaprolactone (low-temperature resistance ≤-58°C), precise chain extension emulsification (VOC <45g / L, Ra <0.2μm), and a gradient curing process (internal stress <10MPa). In the comparative examples, deviations from any single parameter (such as insufficient cross-linking density or excessively short emulsification time) significantly degraded performance, resulting in a 15-50°C drop in the high-temperature softening temperature, an increase in water absorption to 8%, and an increase in internal stress to over 15MPa. Experimental data demonstrates that only by synergistically controlling the cross-linked network structure, soft segment molecular design, and process parameters can environmental performance, durability, and adaptability to extreme environments be achieved.

[0110] The applicant declares that the present invention uses the above-described embodiments to illustrate the detailed process flow of the present invention, but the present invention is not limited to the above-described detailed process flow, that is, it does not mean that the present invention must rely on the above-described detailed process flow to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent replacements for various raw materials in the products of the present invention, addition of auxiliary ingredients, and selection of specific methods, etc., are all within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a high and low temperature resistant environmentally friendly water-based polyurethane coating, characterized in that: The following steps are involved: S1: mixing isophorone diisocyanate and cyanuric acid, and reacting them at 80-90° C. and 250-350 rpm under nitrogen protection for 2.5-3.5 hours to obtain a prepolymer containing a triazine ring cross-linked network, wherein the molar ratio of isophorone diisocyanate to cyanuric acid is 2.8:1 to 3.2:1; S2: adding hyperbranched polycaprolactone to the prepolymer in a mass ratio of the hyperbranched polycaprolactone to the isophorone diisocyanate of 0.45:1, stirring at 200-300 rpm at 60-70° C., and simultaneously dehydrating under vacuum for 2.5-3.5 hours to obtain a mixed prepolymer; S3: adding dimethylolpropionic acid chain extender to the mixed prepolymer, adding triethylamine for neutralization after the reaction, adjusting the pH to 8.5-9.0, then adding deionized water at a shear rate of 4500-5500 rpm for emulsification and dispersion, and adding coating additives to obtain a waterborne polyurethane coating; Wherein, the mass ratio of the mixed prepolymer, the dimethylol propionic acid chain extender and the coating additive is 100:(4-5):(1.3-4.3).

2. The preparation method according to claim 1, characterized in that The reaction temperature of S1 is 83-87° C., the reaction time is 2.8-3.2 h, and the stirring rate is 280-320 rpm.

3. The preparation method according to claim 1, characterized in that The molecular weight of the hyperbranched polycaprolactone in S2 is 3000-4000, and the vacuum degree during vacuum dehydration is -0.090 to -0.098 MPa.

4. The preparation method according to claim 1, characterized in that The molar ratio of the triethylamine to the dimethylolpropionic acid is 1.05-1.15:

1.

5. The preparation method according to claim 1, characterized in that The emulsification and dispersion time of S3 is 25-35 minutes, and the emulsification temperature is 55-65°C.

6. The preparation method according to any one of claims 1 to 5, characterized in that The hyperbranched polycaprolactone is prepared by the following steps: Caprolactone and pentaerythritol are subjected to a ring-opening polymerization reaction at a molar ratio of hydroxyl group to ester group of 1:8-12, and stannous octoate (0.5-1.2% by weight of caprolactone) is added as a catalyst. The reaction is carried out at 120-140° C. under nitrogen protection for 4-6 hours to obtain a hyperbranched polycaprolactone with a branching degree of 15-25% and a hydroxyl value of 45-55 mg KOH / g.

7. A high and low temperature resistant environmentally friendly water-based polyurethane coating, characterized in that: Prepared according to the method according to any one of claims 1 to 6.

8. A method for preparing a high and low temperature resistant environmentally friendly water-based polyurethane coating, characterized in that: include: After applying the high and low temperature resistant environmentally friendly water-based polyurethane coating described in claim 7, it is sequentially subjected to room temperature surface drying, gradient temperature curing and aging treatment to obtain the high and low temperature resistant environmentally friendly water-based polyurethane coating.

9. The preparation method according to claim 8, characterized in that The gradient temperature rising curing comprises: First cure at 75-85℃ for 50-70min, then cure at 115-125℃ for 25-35min; The aging treatment includes: Curing is carried out for 20-28 hours in an environment with a humidity of 40-60%.

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