High and low temperature resistant environment-friendly waterborne polyurethane coating and preparation method thereof

Abstract

The invention discloses a high and low temperature resistant environment-friendly waterborne polyurethane coating and a preparation method thereof, and belongs to the technical field of chemical coatings. The preparation method comprises the following steps: reacting isophorone diisocyanate with cyanuric acid at 80-90 DEG C to generate a prepolymer containing a triazine ring cross-linked network; hyperbranched polycaprolactone is added into the prepolymer, and a mixed prepolymer is formed through vacuum dehydration; the preparation method comprises the following steps: carrying out chain extension by adopting dimethylolpropionic acid, neutralizing by adopting triethylamine, adding a coating additive, shearing and emulsifying to obtain the high and low temperature resistant environment-friendly waterborne polyurethane coating. Through the synergistic effect of a triazine ring rigid network and a hyperbranched chain segment, the coating has brittle fracture resistance and creep resistance in a wide temperature range; the amphiphilic chain extender ensures the stability of the emulsion, and the VOC content is low.

Description

Technical Field

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

[0002] Waterborne polyurethane coatings are widely used in the automotive, electronics, construction and other fields due to their environmental friendliness (low VOC emissions) and excellent mechanical properties. Traditional waterborne polyurethane coatings are usually based on linear polyurethane molecular chains and achieve water dispersibility by introducing hydrophilic groups. However, the temperature resistance of such coatings is limited by the balance between the flexibility of the molecular chain and the cross-linking density, resulting in a narrow long-term use temperature range, which is difficult to meet the application requirements in extreme environments.

[0003] In the existing technology, the low temperature resistance of conventional waterborne polyurethane coatings is generally above -30°C. Under low temperature conditions, the movement of molecular chains is hindered, and the coating is prone to brittle fracture. Under high temperature conditions (over 120°C), the thermal movement of molecular chains intensifies, and the cross-linked network is prone to relaxation, causing the coating to soften or even stick. Although some studies have attempted to improve temperature resistance by adding silicone modification or inorganic fillers, they are often accompanied by increased VOC content or a significant decrease in coating flexibility, making it difficult to balance environmental protection and wide temperature range stability.

[0004] Patent CN105802431A discloses a waterborne polyurethane coating that is modified by grafting amino and isocyanate groups on the surface of diatomite and compositely modified with terminal hydroxyl polybutadiene and acrylic resin. This solution uses inorganic fillers such as montmorillonite and kaolin to enhance mechanical properties, and uses the adsorption characteristics of diatomite to give the coating the function of purifying formaldehyde. However, this method has the following limitations: First, the use of silane coupling agents and toluene diisocyanate may lead to an increase in VOC content, which conflicts with environmental protection requirements; secondly, the introduction of a large amount of inorganic fillers improves high temperature resistance, but it will aggravate the tendency of low-temperature embrittlement, and the compatibility problem of multiphase interfaces easily leads to a decrease in coating uniformity; in addition, the complex multi-step grafting polymerization process (such as diatomite pretreatment, prepolymer synthesis, acrylate copolymerization, etc.) significantly increases production energy consumption and quality control difficulty, making it difficult to achieve industrial and efficient preparation. Therefore, how to balance the temperature resistance, flexibility and environmental protection of the coating in a wider temperature range (-60°C to 160°C) while simplifying the preparation process is still a technical bottleneck that needs to be broken through in this field.

[0005] In addition, in the prior art, excessive hydrophilic chain extenders are often used to improve the stability of the emulsion, resulting in deterioration of the water resistance of the coating film; and high-temperature curing processes (such as single-stage high-temperature treatment) are prone to intensify microphase separation, affecting the uniformity of the coating. Therefore, the development of a waterborne polyurethane coating with excellent mechanical strength, flexibility and low VOC properties in a wide temperature range of -60°C to 160°C is still a technical problem that needs to be solved in this field. SUMMARY OF THE INVENTION

[0006] In view of the deficiencies of the prior art, the present invention provides a preparation method of a high and low temperature resistant environmentally friendly waterborne polyurethane coating, comprising the following steps: S1: Mix isophorone diisocyanate and cyanuric acid, and stir and react at 80 - 90°C at 250 - 350 rpm for 2.5 - 3.5 h under nitrogen protection to obtain a prepolymer containing a triazine ring crosslinking network; S2: Add hyperbranched polycaprolactone to the prepolymer, and the mass ratio of the hyperbranched polycaprolactone to the isophorone diisocyanate is 0.45:1. Stir and react at 60 - 70°C at 200 - 300 rpm, and simultaneously dehydrate under vacuum for 2.5 - 3.5 h to obtain a mixed prepolymer; S3: Add a chain extender of dimethylolpropionic acid to the mixed prepolymer, and after the reaction, add triethylamine for neutralization, adjust the pH to 8.5 - 9.0, and then add deionized water for emulsification and dispersion at a shear rate of 4500 - 5500 rpm, and simultaneously add coating additives to obtain a high and low temperature resistant environmentally friendly waterborne polyurethane coating; wherein, the mass ratio of the mixed prepolymer, the dimethylolpropionic acid chain extender and the coating additives 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 and 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 reaction of caprolactone and pentaerythritol according to the molar ratio of hydroxyl group to ester group of 1:8 - 12, adding stannous octoate accounting for 0.5 - 1.2% of the mass of caprolactone as a catalyst, and reacting at 120 - 140°C under nitrogen protection for 4 - 6 h to obtain hyperbranched polycaprolactone with 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 environmentally friendly waterborne polyurethane coating prepared by any of the foregoing methods.

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

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. By introducing a rigid cross - linked network of the triazine ring of cyanuric acid, the prepolymer forms a high - cross - linked density structure, effectively inhibiting the thermal motion of molecular segments at high temperatures and endowing the cured coating with excellent high - temperature anti - softening ability.

[0016] 2. By grafting hyperbranched polycaprolactone flexible chain segments, dynamic flexible microdomains are formed in the molecular chain, inhibiting the embrittlement caused by the crystallization of segments at low temperatures (-40°C) and enhancing the low - temperature impact resistance of the cured coating.

[0017] 3. Through the gradient temperature curing and aging process, the coating constructs a rigid - flexible interpenetrating network in stages, reducing the concentration of thermal stress and enhancing the dimensional stability in a wide temperature range.

[0018] 4. Through the amphiphilic chain extender and high - shear emulsification process, the polyurethane segments form a stable micelle dispersion, improving the storage stability of the water - borne emulsion and increasing the density of the cured coating.

[0019] 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.

[0020] 6. Through the synergistic control of the molecular weight of hyperbranched polycaprolactone and the dehydration vacuum degree, the compatibility between the hard and soft segments is optimized, and the interfacial bonding strength is improved, avoiding cracking of the cured coating caused by phase separation.

[0021] 7. Through the regulation of the curing temperature and aging humidity in stages, the cross - linked network is perfected in depth, and the thermal - cycling resistance of the cured coating is significantly enhanced.

[0022] 8. Through the precise control of the addition amount of dimethylolpropionic acid and the neutralization molar ratio, the hydrophilic - hydrophobic balance degree and water resistance reach the optimal match. Detailed implementation manners

[0023] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, 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 some, but not all, of the embodiments of this application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in this application without creative efforts shall fall within the scope of protection of this application.

[0024] The present invention provides a method for preparing a high and low temperature resistant environmentally friendly waterborne polyurethane coating, comprising the following steps: S1: Mix isophorone diisocyanate and cyanuric acid, and under nitrogen protection, stir and react at 80 - 90 °C at 250 - 350 rpm for 2.5 - 3.5 h to obtain a prepolymer containing a triazine ring crosslinking network; S2: Add hyperbranched polycaprolactone to the prepolymer. The mass ratio of the hyperbranched polycaprolactone to the isophorone diisocyanate is 0.45:1. Stir and react at 60 - 70 °C at 200 - 300 rpm, and simultaneously dehydrate under vacuum for 2.5 - 3.5 h to obtain a mixed prepolymer; S3: Add a chain extender of dimethylolpropionic acid to the mixed prepolymer. After the reaction, add triethylamine for neutralization, adjust the pH to 8.5 - 9.0, and then add deionized water for emulsification and dispersion at a shear rate of 4500 - 5500 rpm. At the same time, add coating additives to obtain a high and low temperature resistant environmentally friendly waterborne polyurethane coating; Wherein, the mass ratio of the mixed prepolymer, the chain extender of dimethylolpropionic acid, and the coating additives is 100:(4 - 5):(1.3 - 4.3).

[0025] In the S1 stage, the isocyanate group (-NCO) of isophorone diisocyanate (IPDI) and the hydroxyl group (-OH) of cyanuric acid undergo a polycondensation reaction and are connected through a urethane bond (-NHCOO-) to form a prepolymer. The rigid structure of the triazine ring of cyanuric acid is introduced into the main chain of the polyurethane to form a three-dimensional crosslinking network. The reaction temperature of 80 - 90 °C under nitrogen protection accelerates the movement of molecular chains and simultaneously avoids side reactions caused by high temperatures; the stirring rate of 250 - 350 rpm promotes the uniform mixing of monomers, and the reaction time of 2.5 - 3.5 h enables the crosslinking density to reach equilibrium, endowing the coating with the ability to resist softening at high temperatures after curing.

[0026] In the S2 and S3 stages, the branching structure of hyperbranched polycaprolactone (HB-PCL) is grafted through hydroxyl groups with the remaining -NCO groups of the prepolymer, and its mass ratio of 0.45:1 with isophorone diisocyanate forms flexible microdomains in the molecular chain, inhibiting the embrittlement caused by the crystallization of chain segments at low temperatures. Since isocyanate groups are highly sensitive to water and can react with trace amounts of water, vacuum dehydration (-0.095 MPa) removes the possible trace amounts of moisture at 60 - 70 °C to avoid the out-of-control crosslinking caused by the reaction of water with -NCO. Subsequently, the added dimethylolpropionic acid (DMBA) extends the chain through the reaction of its hydroxyl groups with the -NCO of the prepolymer, and its carboxylic acid groups (-COOH) are neutralized by triethylamine to form carboxylates (-COO⁻), making the polyurethane chain segments amphiphilic; high-shear emulsification at 4500 - 5500 rpm enables the hydrophobic chain segments to wrap the hydrophilic groups to form stable micelles, and a pH of 8.5 - 9.0 ensures the full ionization of the carboxylates, achieving aqueous dispersion.

[0027] Through the synergistic effect of introducing a triazine ring crosslinking network and hyperbranched polycaprolactone, the cured coating has both anti-embrittlement and anti-softening properties in a wide temperature range; the design of the amphiphilic chain extender improves the stability of the aqueous emulsion, and at the same time, through the selection of a fully aqueous system and a low-volatility neutralizing agent, the VOC content of the coating is significantly reduced, meeting the environmental protection requirements.

[0028] In some embodiments, the coating additives include a defoamer, a leveling agent, and a film-forming aid with a mass ratio of 1:(2 - 3):(5 - 8).

[0029] In some embodiments, the defoamer is polydimethylsiloxane, the leveling agent is polyether-modified polysiloxane, and the film-forming aid is 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate.

[0030] In some embodiments, the hyperbranched polycaprolactone is prepared by the following steps: Caprolactone and pentaerythritol are subjected to a ring-opening polymerization reaction with a molar ratio of hydroxyl groups to ester groups of 1:8 - 12, and stannous octoate with a mass of 0.5 - 1.2% of the caprolactone is added as a catalyst, and the reaction is carried out at 120 - 140 °C under nitrogen protection for 4 - 6 h to obtain hyperbranched polycaprolactone with a degree of branching of 15 - 25% and a hydroxyl value of 45 - 55 mg KOH / g.

[0031] In some embodiments, the molar ratio of isophorone diisocyanate to cyanuric acid in the S1 is 2.8:1 to 3.2:1.

[0032] When the molar ratio is lower than 3:1 but higher than 2.8:1, the relative excess of cyanuric acid can increase the crosslinking point density of the triazine ring, but sufficient isocyanate groups need to be retained for subsequent chain extension; when it is higher than 3:1 but lower than 3.2:1, the chain extension reaction activity is ensured by the excess isocyanate groups. This ratio range balances the rigidity of the crosslinking network and the efficiency of the chain extension reaction, enabling the prepolymer to form a rigid framework that resists high-temperature softening while retaining sufficient reactive groups to react with subsequent hyperbranched polycaprolactone and chain extenders, avoiding the deterioration of the mechanical properties of the cured coating caused by excessive crosslinking or insufficient chain extension, and at the same time improving the tolerance of the raw material feeding process.

[0033] 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.

[0034] By limiting the reaction temperature to 83 - 87°C, the time to 2.8 - 3.2 h, and the stirring rate to 280 - 320 rpm, an equilibrium is established between the kinetics and thermodynamics of the polycondensation reaction. When the temperature is lower than 83°C, the reaction rate is insufficient, resulting in an incomplete crosslinking network; when it is higher than 87°C, the self-polymerization side reaction of the isocyanate groups is accelerated; the reaction time of 2.8 - 3.2 h ensures the full reaction of the hydroxyl groups of cyanuric acid and avoids the gelation of the prepolymer caused by excessive crosslinking; the stirring rate of 280 - 320 rpm not only ensures the uniform dispersion of the monomers but also avoids the high-speed shear from damaging the triazine ring crosslinking structure. By controlling the reaction process, the molecular weight distribution of the prepolymer is narrowed, reducing the problem of uneven chain segment entanglement in the subsequent chain extension stage and improving the high-temperature creep resistance of the cured coating.

[0035] 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.

[0036] When the molecular weight is lower than 3000, the branched chains are too short, resulting in insufficient flexibility; when it is higher than 4000, the interfacial binding force with the triazine ring network is reduced due to the too long chain segments; when the vacuum degree during vacuum dehydration is too low (>-0.090 MPa), water residues cause subsequent chain extension side reactions, and when it is too high (<-0.098 MPa), the volatilization of low-boiling monomers is accelerated, damaging the ratio. By synergistically controlling the molecular weight matching and the degree of dehydration, the soft segments are evenly interspersed in the rigid network, suppressing low-temperature embrittlement while maintaining high-temperature dimensional stability. By limiting the molecular weight of the hyperbranched polycaprolactone to 3000 - 4000 and the vacuum degree of vacuum dehydration to -0.090 to -0.098 MPa, the compatibility of the hard and soft segments is optimized in the S2 stage.

[0037] In some embodiments, the molar ratio of triethylamine to dimethylolpropionic acid is 1.05 - 1.15:1.

[0038] When the content of dimethylolpropionic acid is less than 4.2%, the lack of carboxylic acid groups leads to poor emulsion dispersion stability, while when it is higher than 4.8%, the excessive hydrophilic chain segments reduce the water resistance of the coating film; the molar ratio of triethylamine of 1.05 - 1.15:1 ensures that the carboxylic acid groups are completely neutralized to form carboxylates (avoiding emulsion flocculation caused by unneutralized acids) and there is no excessive residual free amine (causing an increase in VOC). By precisely controlling the hydrophilic-hydrophobic balance, the emulsion has a uniform particle size and a continuous and dense structure is formed after the coating is cured, taking into account environmental protection and mechanical strength.

[0039] In some embodiments, the emulsification and dispersion time of S3 is 25 - 35 min, and the emulsification temperature is 55 - 65 °C.

[0040] By limiting the emulsification and dispersion time of S3 to 25 - 35 min and the emulsification temperature to 55 - 65 °C, the kinetics of micelle formation and thermal stability are balanced during the shear emulsification process. When the emulsification time is less than 25 min, the high shear force cannot fully break the polyurethane chain segments to form uniform micelles, and when it exceeds 35 min, the formed stable micelle structure may be damaged; since the chain segment rigidity increases at too low a temperature, resulting in difficult emulsification, and too high a temperature causes self-polymerization of unreacted isocyanate groups, the temperature range of 55 - 65 °C can maintain the appropriate mobility of the polyurethane chain segments. By matching the shear time and temperature window, the emulsion particle size distribution is concentrated, avoiding surface roughness of the coating film caused by too large particle size or deterioration of permeability caused by too small particle size, thereby improving the film formation uniformity and adhesion of the coating after the coating is cured.

[0041] The present invention also provides a high and low temperature resistant and environmentally friendly waterborne polyurethane coating prepared by any of the foregoing methods.

[0042] The present invention also provides a method for preparing a high and low temperature resistant and environmentally friendly waterborne polyurethane coating, including: after coating the high and low temperature resistant and environmentally friendly waterborne polyurethane coating, successively performing surface drying at room temperature, gradient heating curing, and aging treatment to obtain the high and low temperature resistant and environmentally friendly waterborne polyurethane coating.

[0043] After the emulsion is coated, the coating structure is constructed in stages through a gradient curing strategy: surface drying at room temperature allows water to evaporate slowly, avoiding film cracking; the initial curing at 80 °C for 1 h promotes the secondary cross-linking of -NCO with residual hydroxyl groups / water to form an initial network; the high temperature treatment at 120 °C for 30 min accelerates the further cross-linking of unreacted groups and eliminates thermal stress; finally, the 24 h aging makes the molecular chains rearrange and densify. The synergistic effect of the triazine ring cross-linking network and the HB-PCL flexible chain maintains the mechanical stability of the coating in a wide temperature range. The gradient curing process enables a dense interpenetrating network to be formed after the coating is cured.

[0044] In some embodiments, the gradient temperature curing includes: first curing at 75 - 85°C for 50 - 70 min, and then curing at 115 - 125°C for 25 - 35 min. The aging treatment includes: aging for 20 - 28 h in an environment with a humidity of 40 - 60%. The 75 - 85°C in the first stage enables the residual isocyanate groups and hydroxyl groups to slowly react to form secondary crosslinking points, forming a preliminary network skeleton; the 115 - 125°C in the second stage activates the deep unreacted groups, promotes the interfacial interpenetration of the triazine ring crosslinking network and the hyperbranched polycaprolactone chain segment, reduces the internal defects of the coating after curing by releasing thermal stress in stages, and at the same time avoids the aggravation of microphase separation caused by single high-temperature curing, thereby enhancing the anti-thermal creep and anti-thermal cycling performance of the coating in a wide temperature range.

[0045] When the humidity is lower than 40%, the environment is too dry, resulting in hindered molecular chain movement and insufficient aging, causing stress concentration in the coating; when it is higher than 60%, water penetration triggers premature hydrolysis of the unreacted groups, destroying the integrity of the crosslinking network; the 20 - 28 h aging time ensures that the molecular chains are fully oriented through Brownian motion, and the rigid network of triazine rings and the hyperbranched flexible chain segments form an interlocking structure through the synergistic effect of humidity and time, thereby enhancing the interfacial bonding strength and fatigue resistance of the coating.

[0046] The method of the present invention will be described in detail below in combination with examples, comparative examples and experimental data.

[0047] Example 1 This example provides a preparation method of a high and low temperature resistant environmentally friendly waterborne polyurethane coating, including the following steps: S1: Add 300 g of isophorone diisocyanate and 42.6 g of cyanuric acid to a reaction kettle, introduce nitrogen for protection, heat up to 85°C and stir and react at 300 rpm for 2.5 h to obtain a prepolymer containing a triazine ring crosslinking network.

[0048] S2: Weigh 135 g of hyperbranched polycaprolactone, add it to the prepolymer obtained in S1, maintain the system temperature at 65°C, stir and react at 250 rpm for 45 min, and then dehydrate for 3 h under a vacuum of -0.095 MPa. After removing water and low-boiling substances, a mixed prepolymer is obtained.

[0049] S3: Cool the mixed prepolymer to 60 °C, add 13.5 g of dimethylolpropionic acid, and after reacting for 1 h, add 10.2 g of triethylamine to adjust the pH to 8.8. Subsequently, at a shear rate of 5000 rpm and an emulsification temperature of 60 °C, add deionized water until the solid content reaches 58%. At the same time, add 1.875 g of polydimethylsiloxane as an antifoaming agent, add 3.75 g of polyether-modified polysiloxane as a leveling agent, add 9.375 g of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate as a film-forming aid, and emulsify and disperse for 30 min to obtain a high and low temperature resistant environmentally friendly waterborne polyurethane coating.

[0050] Among them, hyperbranched polycaprolactone is prepared by the following method: Add caprolactone and pentaerythritol to the reaction kettle according to a molar ratio of hydroxyl group to ester group of 1:10, and then add 0.85% of stannous octanoate based on the mass of caprolactone as a catalyst. After purging the air with nitrogen, heat up to 130 °C and continuously stir and react for 5 h to obtain hyperbranched polycaprolactone with an average molecular weight of 3000 - 4000.

[0051] Example 2 This example provides a preparation method of a high and low temperature resistant environmentally friendly waterborne polyurethane coating, including the following steps: S1: Add 300 g of isophorone diisocyanate and 40.0 g of cyanuric acid to the reaction kettle, protect with nitrogen, heat up to 90 °C and stir and react at 350 rpm for 3.0 h to obtain a prepolymer containing a triazine ring cross-linked network.

[0052] S2: Weigh 135 g of hyperbranched polycaprolactone and add it to the prepolymer obtained in S1. Maintain the system temperature at 60 °C, stir and react at 200 rpm for 30 min, and then dehydrate at a vacuum of -0.098 MPa for 3.5 h. After removing water and low-boiling substances, a mixed prepolymer is obtained.

[0053] S3: Cool the mixed prepolymer to 60 °C, add 13.8 g of dimethylolpropionic acid, and after reacting for 1 h, add 10.5 g of triethylamine to adjust the pH to 8.8. Subsequently, at a shear rate of 5000 rpm and an emulsification temperature of 65 °C, add deionized water until the solid content reaches 58%. At the same time, add 1.875 g of polydimethylsiloxane as an antifoaming agent, add 3.75 g of polyether-modified polysiloxane as a leveling agent, add 9.375 g of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate as a film-forming aid, and emulsify and disperse for 35 min to obtain a high and low temperature resistant environmentally friendly waterborne polyurethane coating.

[0054] Among them, the hyperbranched polycaprolactone is prepared by the following method: Caprolactone and pentaerythritol are added to the reaction kettle according to a molar ratio of hydroxyl group to ester group of 1:12, and then stannous octoate accounting for 0.5% of the mass of caprolactone is added as a catalyst. After purging the air with nitrogen, the temperature is raised to 140 °C, and the reaction is continuously stirred for 6 h to obtain hyperbranched polycaprolactone with an average molecular weight of 3000-4000.

[0055] Example 3 This example provides a preparation method of a high and low temperature resistant environmentally friendly waterborne polyurethane coating, including the following steps: S1: 300 g of isophorone diisocyanate and 45.5 g of cyanuric acid are added to the reaction kettle, protected by nitrogen, heated to 80 °C and stirred at 250 rpm for 3.5 h to obtain a prepolymer containing a triazine ring crosslinked network.

[0056] S2: Weigh 135 g of hyperbranched polycaprolactone and add it to the prepolymer obtained in S1. Maintain the system temperature at 60 °C and stir at 300 rpm for 60 min. Subsequently, dehydrate for 2.5 h under a vacuum of -0.090 MPa to remove moisture and low-boiling substances, and obtain a mixed prepolymer after removal.

[0057] S3: Cool the mixed prepolymer to 60 °C, add 12.6 g of dimethylolpropionic acid, and after reacting for 1 h, add 9.8 g of triethylamine to adjust the pH to 8.8. Subsequently, add deionized water to a solid content of 58% at a shear rate of 5000 rpm and an emulsification temperature of 55 °C. At the same time, add 1.875 g of polydimethylsiloxane as an antifoaming agent, add 3.75 g of polyether-modified polysiloxane as a leveling agent, add 9.375 g of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate as a film-forming aid, and emulsify and disperse for 25 min to obtain a high and low temperature resistant environmentally friendly waterborne polyurethane coating.

[0058] Among them, the hyperbranched polycaprolactone is prepared by the following method: Caprolactone and pentaerythritol are added to the reaction kettle according to a molar ratio of hydroxyl group to ester group of 1:8, and then stannous octoate accounting for 1.2% of the mass of caprolactone is added as a catalyst. After purging the air with nitrogen, the temperature is raised to 120 °C, and the reaction is continuously stirred for 4 h to obtain hyperbranched polycaprolactone with an average molecular weight of 3000-4000.

[0059] Comparative Example 1 This comparative example provides a preparation method of a high and low temperature resistant environmentally friendly waterborne polyurethane coating, which is different from that of Example 1 in that: in S1, 300 g of isophorone diisocyanate and 64.5 g of cyanuric acid are used.

[0060] Comparative Example 2 This comparative example provides a method for preparing a high and low temperature resistant environmentally friendly waterborne polyurethane coating, which is different from Example 1 in that cyanuric acid is not added in S1, and in S2, only isophorone diisocyanate and hyperbranched polycaprolactone are reacted.

[0061] Comparative Example 3 This comparative example provides a method for preparing a high and low temperature resistant environmentally friendly waterborne polyurethane coating, which is different from Example 1 in that hyperbranched polycaprolactone is not added in S2, and is replaced with linear polycaprolactone (Mn = 3500) of equal mass.

[0062] Comparative Example 4 This comparative example provides a method for preparing a high and low temperature resistant environmentally friendly waterborne polyurethane coating, which is different from Example 1 in that the addition amount of dimethylolpropionic acid in S3 is 3.5% of the total mass of the prepolymer.

[0063] Comparative Example 5 This comparative example provides a method for preparing a high and low temperature resistant environmentally friendly waterborne polyurethane coating, which is different from Example 1 in that the emulsification and dispersion time in S3 is 15 min and the emulsification temperature is 45 °C.

[0064] Comparative Example 6 This comparative example provides a method for preparing a high and low temperature resistant environmentally friendly waterborne polyurethane coating, which is different from Example 1 in that the vacuum degree for vacuum dehydration in S2 is -0.080 MPa and the dehydration time is 1 h.

[0065] The properties of the high and low temperature resistant environmentally friendly waterborne polyurethane coatings provided in the above examples and comparative examples were tested. First, a high and low temperature resistant environmentally friendly waterborne polyurethane coating was prepared according to the following method: The high and low temperature resistant environmentally friendly waterborne polyurethane coating was applied on the surface of the substrate. After the surface was dry at room temperature for 30 min, it was first cured at 80 °C for 1 h, then heated to 120 °C and cured for 30 min, and finally aged in an environment with a humidity of 50% for 24 h to obtain a 50-μm-thick coating.

[0066] Then the coating was tested, and the test methods were as follows: 1. High temperature softening temperature: Measured by a thermomechanical analyzer (TMA, TA Instruments Q400). The coating sample (10×10×1 mm³) was heated from 25 °C to 200 °C at a heating rate of 5 °C / min, and the temperature value corresponding to 1% strain on the deformation curve was recorded.

[0067] 2. Low temperature crack resistance temperature: The coating sample (100×25×0.05 mm³) was placed in a high and low temperature test chamber (ESPECS U-221), cooled to the target temperature (-70 °C to 25 °C) at a rate of 10 °C / min, and after holding for 30 min, a 180° bending test was carried out, and the lowest temperature at which surface cracks occurred was observed.

[0068] 3. VOC Content: Determined according to the standard of GB 38508-2020 using a gas chromatography-mass spectrometry (Agilent 7890B / 5977B). Take 1 g of the emulsion sample, heat it at 120 °C for 1 h, capture volatile organic compounds, and quantitatively calculate the total VOC content by the internal standard method (toluene as the internal standard).

[0069] 4. Water Absorption Rate: Immerse the coating sample (50×50×0.05 mm³) in deionized water at 25 °C, take it out after 240 h, dry the surface moisture, and weigh the mass change: Water Absorption Rate (%) = (mass after immersion - initial mass) / initial mass × 100%.

[0070] 5. Adhesion (Cross-Cut Method): According to the ISO 2409 standard, use a six-blade cutter (blade spacing 2 mm) to draw a 6×6 grid on the coating surface, paste 3M 610 tape, and quickly peel it off at a 60° angle. Evaluate the grade according to the proportion of the peeling area (Grade 0: no peeling; Grade 1: ≤5%; Grade 2: 5 - 15%).

[0071] 6. Thermal Cycling Test: Cycle the coating sample 10 times between -60 °C (30 min) and 160 °C (30 min), with the conversion time < 5 min each time. After the cycle, use an optical microscope (Olympus BX53) to observe the surface cracks and peeling conditions.

[0072] 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 mean roughness Ra value (measurement resolution 0.1 nm).

[0073] 8. Internal Stress: Adopt the substrate bending method, use a laser displacement sensor (Keyence LK-G5000) to measure the change in the curvature radius of the substrate before and after curing, and calculate the internal stress according to the following formula: σ = Es·ts² / (6(1 - vs)·tf)·(1 / Rf - 1 / Ri) Where Es = 160 GPa (elastic modulus of the silicon wafer), 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 curing.

[0074] The above performance test data are shown in Table 1.

[0075] Table 1 Performance Test Results

[0076] Analysis of Experimental Results In Example 1, the coating has no cracks or softening in the range of -60°C to 160°C, the VOC content is 42 g / L, the water absorption rate is 2.8%, the adhesion is grade 0, the surface roughness Ra = 0.15 μm, and the internal stress is 8 MPa. Its wide-temperature stability benefits from the synergistic effect of the triazine ring cross-linked network and hyperbranched polycaprolactone. Gradient curing and sufficient curing make the coating dense and defect-free.

[0077] In Example 2, the high-temperature softening temperature slightly drops to 158°C, the low-temperature crack resistance is -58°C, the VOC is 45 g / L, the water absorption rate is 3.1%, indicating that the performance can still be maintained under the process limit conditions, but the surface roughness (Ra = 0.18 μm) slightly increases, which may be related to the slight gelation of the prepolymer at higher temperatures.

[0078] In Example 3, the low-temperature crack resistance temperature is increased to -62°C, but the high-temperature softening temperature rises to 162°C, and the water absorption rate is 2.5%, indicating that lower cross-linking density and more sufficient dehydration can optimize the low-temperature resistance, but the high-temperature stability needs to be balanced.

[0079] In Comparative Example 1, the high-temperature softening temperature is only 135°C, and it cracks at -40°C, and the VOC is 50 g / L, indicating that insufficient cross-linking network density leads to deterioration of heat resistance, and the increase in unreacted isocyanate groups causes side reactions (increase in VOC).

[0080] In Comparative Example 2, the high-temperature softening temperature is 110°C, the adhesion is grade 2, and it peels severely after thermal cycling, proving that the absence of the triazine ring makes the coating lose its high-temperature creep resistance, and the interfacial bonding force decreases significantly.

[0081] In Comparative Example 3, the low-temperature crack resistance temperature is only -40°C, the high-temperature modulus drops by 50%, and the surface roughness Ra = 0.25 μm, indicating that the linear chain segments cannot inhibit low-temperature crystallization and have poor compatibility with the rigid network.

[0082] In Comparative Example 4, the VOC rises to 68 g / L, the water absorption rate is 7.5%, and emulsion stratification leads to fish-eye defects in the coating film, confirming that insufficient chain extender requires additional solvent, and too few hydrophilic groups affect the emulsion stability.

[0083] In Comparative Example 5, the surface roughness Ra = 1.2 μm, the water absorption rate is 8%, and the emulsion particle size D90 > 500 nm, indicating that insufficient emulsification leads to loose micelle structure and deteriorated film permeability.

[0084] In Comparative Example 6, the water content > 0.5% causes the emulsion viscosity to fluctuate by ±30%, and the coating film foams after being immersed in water for 48 h, verifying the influence of incomplete dehydration on water resistance.

[0085] Examples 1-3 achieved a high-stability coating with a wide temperature range (-60~160°C) through a triazine ring crosslinked network (high temperature resistance ≥158°C), hyperbranched polycaprolactone (low temperature resistance ≤-58°C), precise chain extension emulsification (VOC < 45 g / L, Ra < 0.2 μm), and gradient curing process (internal stress < 10 MPa). Any deviation of a single parameter in the comparative examples (such as insufficient crosslinking density or too short emulsification time) led to a significant deterioration in performance. For example, the high-temperature softening temperature decreased by 15-50°C, the water absorption rate increased to 8%, and the internal stress increased to over 15 MPa. Experimental data proved that only by synergistically controlling the crosslinked network structure, soft segment molecular design, and process parameters could environmental friendliness, durability, and extreme environment adaptability be taken into account.

[0086] The applicant declares that the detailed process flow of the present invention is illustrated by the above examples, but the present invention is not limited to the above detailed process flow, that is, it does not mean that the present invention must rely on the above detailed process flow to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of each raw material of the product of the present invention, the addition of auxiliary components, and the selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A preparation method of a high and low temperature resistant environmental protection waterborne polyurethane coating, characterized in that, It includes the following steps: S1: Mix isophorone diisocyanate and cyanuric acid, and under nitrogen protection, stir and react at 80 - 90 °C at 250 - 350 rpm for 2.5 - 3.5 h to obtain a prepolymer containing a triazine ring cross - linked network; S2: Add hyperbranched polycaprolactone to the prepolymer. The mass ratio of the hyperbranched polycaprolactone to the isophorone diisocyanate is 0.45:

1. Stir and react at 60 - 70 °C at 200 - 300 rpm, and simultaneously dehydrate under vacuum for 2.5 - 3.5 h to obtain a mixed prepolymer; S3: Add a chain extender of dimethylolpropionic acid to the mixed prepolymer. After reaction, add triethylamine for neutralization, adjust the pH to 8.5 - 9.0, and then add deionized water for emulsifying dispersion at a shear rate of 4500 - 5500 rpm. Meanwhile, add coating auxiliaries to obtain a waterborne polyurethane coating; Wherein, the mass ratio of the mixed prepolymer, the dimethylolpropionic acid chain extender to the coating auxiliaries is 100:(4 - 5):(1.3 - 4.3).

2. The preparation method according to claim 1, wherein In S1, the molar ratio of isophorone diisocyanate to cyanuric acid is 2.8:1 to 3.2:

1.

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

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

5. 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.

6. The preparation method according to claim 1, characterized in that, The emulsifying dispersion time in S3 is 25 - 35 min, and the emulsifying temperature is 55 - 65 °C.

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

8. An environmentally friendly waterborne polyurethane coating resistant to high and low temperatures, characterized in that, Prepared according to the method described in any one of claims 1 - 7.

9. A preparation method of a high and low temperature resistant environmental protection waterborne polyurethane coating, characterized in that, It includes: After coating the high and low temperature resistant environmentally friendly waterborne polyurethane coating described in claim 8, carry out surface drying at room temperature, gradient temperature curing and aging treatment in sequence to obtain the high and low temperature resistant environmentally friendly waterborne polyurethane coating.

10. The preparation method according to claim 9, characterized in that The gradient temperature curing includes: First cure at 75 - 85 °C for 50 - 70 min, and then cure at 115 - 125 °C for 25 - 35 min; The aging treatment includes: Carry out aging for 20 - 28 h in an environment with a humidity of 40 - 60%.

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