A composite curing agent and its preparation method and its application in low TDI content polyurethane.

Abstract

This invention provides a composite curing agent, its preparation method, and its application in low-TDI polyurethane, relating to the field of polyurethane curing agent technology. The preparation method includes: reacting isophorone diisocyanate with a polyester polyol in an anhydrous solvent at 70℃-80℃ to obtain a flexible prepolymer; reacting toluene diisocyanate with molten trimethylolpropane at 60℃-80℃ to obtain a rigid prepolymer; reacting the flexible and rigid prepolymers at 65℃-75℃ to obtain a composite precursor; end-capping the composite precursor at 45℃-55℃ to obtain an end-capped intermediate; and grafting the end-capped intermediate with hexamethylene diisocyanate trimer at 70℃-80℃ under the action of a catalyst to obtain a composite curing agent. This invention can effectively convert free TDI monomers, reduce the free TDI content in the curing agent, and simultaneously improve the hardness and adhesion of the polyurethane coating.

Description

Technical Field

[0001] This invention relates to the field of polyurethane curing agent technology, and in particular to a composite curing agent, its preparation method, and its application in polyurethane with low TDI content. Background Technology

[0002] Polyurethane coatings are a commonly used high-performance coating material. Due to the excellent abrasion resistance, chemical corrosion resistance, and high gloss of the cured polyurethane coating, they are widely used in the automotive, furniture, and construction industries. Currently, commonly used polyurethane coatings are two-component polyurethanes, specifically consisting of a polyol resin containing terminal hydroxyl groups (-OH) and a curing agent containing isocyanate groups (-NCO). The curing agent undergoes an addition polymerization reaction with the polyol resin to form urethane bonds (-NH-COO-), thereby forming a dense polyurethane cross-linked network. This achieves rapid curing of the coating film and enhances its mechanical properties and durability.

[0003] Toluene diisocyanate (TDI) is a commonly used raw material in the synthesis of curing agents. Its two isocyanate groups possess extremely high reactivity, which helps improve reaction efficiency and shorten the reaction cycle during curing agent synthesis. Furthermore, the rigid benzene ring structure in the TDI molecule can be introduced into the macromolecular network of the polyurethane coating during film formation, thereby improving the coating's hardness and curing rate. However, TDI is a highly volatile organic compound with significant biotoxicity. When inhaled or absorbed through the skin, it can easily cause respiratory irritation, allergies, and even pose a carcinogenic risk.

[0004] Therefore, to reduce the free TDI content in polyurethane, two common methods are currently used: one is to use TDI prepolymers or adducts to reduce the volatilization of free TDI; the other is to replace TDI with other isocyanates to reduce the proportion of TDI in the curing agent. However, both methods have significant limitations. First, although the first method can reduce the free TDI content, the residual TDI content is still relatively high and will directly reduce the curing rate of the coating. The second method, on the other hand, easily reduces the mechanical strength of the coating. Therefore, there is an urgent need to provide a solution to improve these problems. Summary of the Invention

[0005] The purpose of this invention is to provide a composite curing agent and its preparation method, as well as its application in polyurethane with low TDI content. By stepwise synthesis and chemical end-capping, free TDI monomers can be effectively converted, thereby effectively reducing the free TDI content in the curing agent. At the same time, by using rigid prepolymers and flexible prepolymers to induce self-assembly to form a composite polymer, the synergy between rigid and flexible segments can be achieved, thereby improving the hardness and flexibility of the polyurethane coating. In addition, by grafting HDI trimer, the weather resistance of the polyurethane coating can be enhanced.

[0006] In a first aspect, the present invention provides a method for preparing a composite curing agent, comprising: reacting isophorone diisocyanate with polyester polyol in an anhydrous solvent at 70°C-80°C to obtain a flexible prepolymer; reacting toluene diisocyanate with molten trimethylolpropane at 60°C-80°C to obtain a rigid prepolymer; reacting the flexible prepolymer and the rigid prepolymer at 65°C-75°C to obtain a composite precursor; end-capping the composite precursor at 45°C-55°C to obtain an end-capping intermediate; and grafting the end-capping intermediate with hexamethylene diisocyanate trimer at 70°C-80°C under the action of a catalyst to obtain a composite curing agent.

[0007] Preferably, the anhydrous solvent includes anhydrous butyl acetate.

[0008] Preferably, the molar ratio of isophorone diisocyanate to polyester polyol is (1.8-2.3):1.

[0009] Preferably, isophorone diisocyanate and polyester polyol are mixed and reacted in an anhydrous protective atmosphere.

[0010] Preferably, isophorone diisocyanate and polyester polyol are pre-dehydrated and dried.

[0011] Preferably, the flexible prepolymer is obtained after mixing and reacting for 3-5 hours.

[0012] Preferably, the polyester polyol includes polycaprolactone diol.

[0013] Preferably, when isophorone diisocyanate and polyester polyol are mixed and reacted, the reaction endpoint is considered to be reached when the deviation of the isocyanate content in the system from the theoretical value is less than or equal to 5%.

[0014] Preferably, the molar ratio of isocyanate in toluene diisocyanate to hydroxyl group in trimethylolpropane is (3-3.2):1.

[0015] Preferably, toluene diisocyanate is dehydrated and dried with trimethylolpropane.

[0016] Preferably, molten trimethylolpropane is added dropwise to toluene diisocyanate.

[0017] Preferably, the toluene diisocyanate includes one of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate.

[0018] Preferably, toluene diisocyanate is mixed with molten trimethylolpropane and reacted for 1-3 hours.

[0019] Preferably, toluene diisocyanate is reacted with molten trimethylolpropane at a stirring speed of 500 rpm to 800 rpm.

[0020] Preferably, the rigid prepolymer is added dropwise to the flexible prepolymer.

[0021] Preferably, the molar ratio of isocyanate groups in the flexible prepolymer to isocyanate groups in the rigid prepolymer is 1:(2-4).

[0022] Preferably, the flexible prepolymer and the rigid prepolymer are mixed and reacted at a rotation speed of 300 rpm to 500 rpm.

[0023] Preferably, the flexible prepolymer and the rigid prepolymer are mixed and reacted for 1-3 hours.

[0024] Preferably, before capping the composite precursor, the content of free toluene diisocyanate monomer in the composite precursor is detected, and then the composite precursor and capping agent are mixed and the capping reaction is carried out at 45℃-55℃.

[0025] Preferably, the capping agent includes diethylamine.

[0026] Preferably, the molar ratio of the capping agent to the free toluene diisocyanate monomer in the composite precursor is (1-1.2):1.

[0027] Preferably, the capping agent is diluted in advance to prepare a capping solution, and then the capping solution is added dropwise to the composite precursor.

[0028] Preferably, the composite precursor and the capping agent are mixed and reacted for 30-60 minutes.

[0029] Preferably, the catalyst includes one of dibutyltin disilicate and stannous octoate.

[0030] Preferably, the mass ratio of catalyst to end-capping intermediate is (0.01-0.1):100.

[0031] Preferably, the catalyst is prepared into a catalytic solution in advance, and then the catalytic solution is mixed with the end-capping intermediate to obtain the catalytic system.

[0032] Preferably, the end-capping intermediate is mixed with the hexamethylene diisocyanate trimer and reacted for 2-4 hours.

[0033] Preferably, the mass ratio of the end-capping intermediate to hexamethylene diisocyanate is 1:(0.12-0.18).

[0034] Preferably, after the mixed grafting, the solid content is adjusted after removing volatile impurities in a vacuum environment at 70℃-80℃.

[0035] Secondly, the present invention also provides a composite curing agent prepared by any of the above-mentioned optional preparation methods, wherein the content of free toluene diisocyanate monomer in the composite curing agent is less than or equal to 0.2 wt%, and the solid content is 75 wt%-77 wt%.

[0036] Thirdly, the present invention also provides the application of a composite curing agent prepared by any of the above-mentioned optional preparation methods in polyurethane with low TDI content.

[0037] Compared to existing technologies, it has at least the following beneficial technical effects:

[0038] 1. By reacting isophorone diisocyanate (IPDI) and toluene diisocyanate (TDI) with polyester polyol and trimethylolpropane respectively to synthesize flexible prepolymers and rigid prepolymers, competitive side reactions caused by differences in the reactivity of different isocyanate monomers can be avoided. At the same time, by utilizing chemical end-capping to preferentially react with free TDI monomers with small molecular weight and high reactivity in the mixed system, free TDI monomers in the mixed system can be removed in a targeted manner, which significantly reduces the toxicity and volatility of the curing agent. In addition, due to the low content of free monomers, the hardness and flexibility of the polyurethane coating can be improved.

[0039] 2. By utilizing the difference in molecular chain structure between flexible and rigid prepolymers, the molecular chain segments of the flexible prepolymer spontaneously wrap and spread on the surface of the rigid prepolymer in a heating environment, thereby causing micro-phase separation and forming a stable composite structure. In the process of polyurethane coating curing, the rigid prepolymer can be used as a mechanical skeleton to effectively improve the hardness and modulus of the polyurethane coating, while also helping to increase the curing rate of the polyurethane coating. In addition, the flexible prepolymer on the surface of the rigid prepolymer can effectively improve the compatibility between the curing agent and the hydroxyl resin, which helps to improve the toughness and impact resistance of the coating.

[0040] 3. By grafting the end-capping intermediate with hexamethylene triisocyanate (HDI) trimer, the highly reactive isocyanate groups on the HDI trimer can react with the less reactive isocyanate groups remaining on the end-capping intermediate under catalysis. This allows the HDI trimer to be grafted onto the composite structure in a chemically bonded manner. This not only further improves the stability of the composite structure, but also helps the curing agent and hydroxyl resin form a denser and more stable three-dimensional network during the polyurethane curing process. Furthermore, it can improve the mechanical strength, hardness, and wear resistance of the coating. Attached Figure Description

[0041] Figure 1 The flowchart illustrates a method for preparing a composite curing agent according to the present invention. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0043] See Figure 1 This invention provides a method for preparing a composite curing agent, comprising the following steps:

[0044] S1. A flexible prepolymer is prepared by mixing and reacting isophorone diisocyanate with polyester polyol in an anhydrous solvent at 70℃-80℃.

[0045] S2. A rigid prepolymer is prepared by reacting toluene diisocyanate with molten trimethylolpropane at 60℃-80℃.

[0046] S3. The composite precursor is prepared by mixing and reacting the flexible prepolymer and the rigid prepolymer at 65℃-75℃.

[0047] S4. At 45℃-55℃, the composite precursor is end-capped to obtain an end-capped intermediate.

[0048] S5. A composite curing agent is prepared by grafting the end-capping intermediate with hexamethylene diisocyanate trimer at 70℃-80℃ under the action of a catalyst.

[0049] In fact, during the reaction of isophorone diisocyanate (IPDI) with polyester polyol in step S1, the isocyanate group (-NCO) on IPDI undergoes nucleophilic addition with the hydroxyl group (-OH) on the polyester polyol. Specifically, the hydroxyl group at the end of the polyester polyol molecule attacks the central carbon atom of the highly electron-deficient isocyanate group on the IPDI molecule. The π electron pair of the N=C double bond transfers to the oxygen atom to form an oxygen anion intermediate, which then rapidly rearranges to form a hydroxyl group. Simultaneously, the electron pair transfers to the nitrogen atom to form a stable urethane bond (-NH-CO-O-).

[0050] In some embodiments, the anhydrous solvent used in step S1 includes anhydrous butyl acetate, and before performing step S1, IPDI and the polyester polyol can be pre-dehydrated and dried, so that the IPDI and polyester polyol react in an anhydrous, protective atmosphere, avoiding hydrolysis of IPDI and polyester polyol in water and the occurrence of side reactions. Specifically, the polyester polyol used in step S1 includes polycaprolactone diol, specifically PCL-1000.

[0051] In some embodiments, during step S1, IPDI and polyester polyol are mixed at a molar ratio of (1.8-2.3):1 and reacted for 3-5 hours. In practice, controlling the amount of IPDI in excess helps to form a flexible segmental prepolymer (IPDI-polyester segment-IPDI) with one IPDI molecule attached to each end. This allows for the encapsulation of the polyester segment with IPDI, thereby improving the compatibility of the flexible prepolymer with the hydroxyl resin through the combined effect of the alicyclic structure of IPDI and the long polyester chain.

[0052] In some embodiments, during step S1, when isophorone diisocyanate and polyester polyol are mixed and reacted, the reaction endpoint is considered reached when the deviation of the isocyanate content in the system from the theoretical value is less than or equal to 5%. In practice, the theoretical value is calculated based on the feed ratio of IPDI to polyester polyol, and the calculated theoretical value is the remaining isocyanate content in the system when the reaction is complete. Specifically, samples can be taken periodically during the mixing reaction, and the isocyanate content in the system can be determined by titration.

[0053] In fact, during the reaction of toluene diisocyanate (TDI) with molten trimethylolpropane (TMP) in step S2, nucleophilic addition also occurs to form stable carbamate bonds. Furthermore, since trimethylolpropane has three individual hydroxyl groups, the molar ratio of isocyanate in toluene diisocyanate to hydroxyl groups in trimethylolpropane can be set to (3-3.2):1 in step S2. This allows one TMP molecule to link three TDI molecules, thereby forming branching points and generating a branched rigid prepolymer with isocyanate end groups.

[0054] In some embodiments, toluene diisocyanate and trimethylolpropane are pre-dehydrated and dried before performing step S2, and the reaction is also carried out in an anhydrous and dry protective atmosphere. Specifically, during step S2, molten trimethylolpropane can be added dropwise to toluene diisocyanate while stirring is maintained during the addition to promote uniform dispersion of TMP in TDI to form a rigid prepolymer. In addition, the reaction between TMP and TDI is highly exothermic, so the reaction temperature can be controlled at 60°C-80°C in a thermostat to avoid side reactions such as dimerization or trimerization of TDI.

[0055] In some embodiments, the toluene diisocyanate used in step S2 includes one of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate. Specifically, in step S2, TMP is added dropwise to TDI, and the mixture is stirred at 500 rpm to 800 rpm for 1 to 3 hours to ensure that TMP and TDI react fully to obtain a rigid prepolymer.

[0056] In fact, during the mixing reaction of the rigid prepolymer and the flexible prepolymer in step S3, the rigid prepolymer (TDI-TMP) exhibits a coiled structure in the anhydrous solvent of the flexible prepolymer due to its rigid benzene rings, difficult molecular chain segment movement, and high polarity. Conversely, the flexible prepolymer (IPDI-polyester segment-IPDI) has compliant polyester segments and relatively low polarity. Therefore, during the mixing reaction, the flexible prepolymer spontaneously migrates to the surface of the rigid prepolymer and extends to encapsulate it, thus forming a composite structure. Furthermore, in the composite structure, the isocyanate groups at the ends of the rigid prepolymer and the flexible prepolymer attract each other, effectively improving the stability of the composite structure.

[0057] In some embodiments, during step S3, the rigid prepolymer can be added dropwise to the flexible prepolymer, thereby creating a reaction environment with an excess of flexible prepolymer, which is beneficial for the flexible prepolymer to coat the surface of the rigid prepolymer to form a composite structure. Furthermore, during the dropwise addition of the rigid prepolymer, stirring at 300-500 rpm can be maintained to promote rapid dispersion of the rigid prepolymer. After the dropwise addition is completed, the stirring speed is maintained for 1-3 hours to ensure a complete reaction. Specifically, the total amount of rigid prepolymer added during step S3 is such that the molar ratio of isocyanate groups in the flexible prepolymer to that in the rigid prepolymer is 1:(2-4).

[0058] In practice, before performing step S4 to cap the composite precursor, the content of free toluene diisocyanate monomer in the composite precursor is measured. Then, the composite precursor is mixed with the capping agent and the capping reaction is carried out at 45℃-55℃. Specifically, the molar ratio of the capping agent to the free toluene diisocyanate monomer in the composite precursor is (1-1.2):1. Further, the capping agent used includes diethylamine.

[0059] In some embodiments, to improve the dispersion uniformity of the capping agent in the composite precursor, the capping agent can be pre-diluted into a capping solution before being added dropwise to the composite precursor. Furthermore, the capping solution can be continuously stirred during the dropwise addition process to fully improve the dispersion performance of the capping agent in the composite precursor and increase the capping reaction efficiency. Specifically, in step S4, the composite precursor and capping agent can be mixed and reacted for 30-60 minutes.

[0060] In some embodiments, the catalyst used in step S5 includes one of dibutyltin dihexylsilicate and stannous octoate, and the mass ratio of the catalyst to the end-capping intermediate is (0.01-0.1):100. In practice, to improve the dispersibility of the catalyst in the end-capping intermediate, the catalyst can be pre-dissolved in anhydrous butyl acetate to prepare a catalytic solution, which is then added dropwise to the end-capping intermediate to mix and obtain the catalytic system.

[0061] In some embodiments, after performing step S5, the end-capping intermediate and hexamethylene diisocyanate trimer are mixed and reacted at a mass ratio of 1:(0.12-0.18) for 2-4 hours, and then volatile impurities are removed under vacuum at 70-80°C before adjusting the solid content. Specifically, the content of free toluene diisocyanate monomer in the obtained composite curing agent is less than or equal to 0.2 wt%, and the solid content is 75 wt%-77 wt%.

[0062] Example 1

[0063] This embodiment 1 provides a method for preparing a composite curing agent, including the following steps:

[0064] S1. Isophorone diisocyanate (IPDI, purchased from Wanhua Chemical Group Co., Ltd.) and polycaprolactone diol (PCL-1000, purchased from Greenlink (Jining) Chemical Technology Co., Ltd.) were dried and dehydrated separately in a vacuum environment at 50°C. Under a nitrogen atmosphere, PCL-1000 was added to anhydrous butyl acetate and stirred and mixed in a water bath at 40°C. Then, IPDI was added dropwise to the reaction system, controlling the total amount of IPDI added to the molar ratio of PCL-1000 to 2.1:1. After the addition was completed, the temperature was raised to 75°C and the reaction was stirred at 300 rpm. During the reaction, samples were taken every 30 minutes to titrate and measure the isocyanate content. When the deviation of the isocyanate content from the theoretical value was less than or equal to 5%, the reaction was considered complete and the reaction time was recorded as 4 hours and 10 minutes. After the reaction was completed, a flexible prepolymer was obtained.

[0065] S2. Trimethylolpropane (CAS: 77-99-6, TMP) was heated to 70°C in a vacuum environment to obtain molten trimethylolpropane. Under a nitrogen atmosphere, molten TMP was added dropwise to toluene diisocyanate (purchased from Shandong Jiaxu Chemical Co., Ltd., TDI) preheated at 70°C. During the dropwise addition, the mixture was stirred at 600 rpm, and the total amount added was controlled so that the molar ratio of isocyanate in TDI to hydroxyl groups in TMP was 3.1:1. After the dropwise addition was completed, the mixture was kept at 70°C and stirred for 2 hours to obtain a rigid prepolymer.

[0066] S3. Under a nitrogen atmosphere at 70°C, the rigid prepolymer is added dropwise to the flexible prepolymer. During the addition process, the stirring speed is maintained at 300 rpm, and the total addition time is controlled to be 30 min. After the addition is completed, the molar ratio of isocyanate groups in the flexible prepolymer to that in the rigid prepolymer is 1:3. The composite precursor is obtained after stirring at 300 rpm for 2 h at 70°C.

[0067] S4. After cooling the composite precursor to 50℃ and holding it at that temperature for 1 hour, the content of free TDI monomer in the composite precursor was detected. Based on the molar ratio of diethylamine (CAS: 109-89-7) to free TDI monomer of 1.1:1, diethylamine was pre-mixed and diluted in anhydrous butyl acetate to prepare a capping solution. The capping solution was then added dropwise to the composite precursor, with the addition time controlled at 30 min. After the addition was completed, the mixture was kept at that temperature and stirred for 45 min to obtain the capping intermediate.

[0068] S5. Based on a mass ratio of dibutyltin disilicate to the end-capping intermediate of 0.05:100, dibutyltin disilicate was dissolved in anhydrous butyl acetate to prepare a catalytic solution. The catalytic solution was then added to the end-capping intermediate and stirred to prepare a catalytic system. Based on a mass ratio of the end-capping intermediate to hexamethylene diisocyanate trimer (purchased from Wuhan Kemike Biomedical Technology Co., Ltd., HDI-100) of 1:0.15, the HDI trimer was added dropwise to the catalytic system. After the addition was completed, the system was kept at 75°C for 3 hours. After removing volatile impurities in a vacuum environment at 75°C, anhydrous butyl acetate was added to adjust the solid content to 76 wt% to obtain a composite curing agent.

[0069] Comparative Example 1

[0070] Comparative Example 1 provides a method for preparing a composite curing agent, comprising the following steps:

[0071] D1. Trimethylolpropane (CAS: 77-99-6, TMP) was melted at 70°C in a vacuum environment to obtain molten trimethylolpropane. Under a nitrogen atmosphere, molten TMP was added dropwise to toluene diisocyanate (purchased from Shandong Jiaxu Chemical Co., Ltd., TDI) preheated at 70°C. During the dropwise addition, the mixture was stirred at 600 rpm, and the total amount added was controlled so that the molar ratio of isocyanate in TDI to hydroxyl groups in TMP was 3.1:1. After the dropwise addition was completed, the mixture was kept at 70°C and stirred for 2 hours to obtain a rigid prepolymer.

[0072] D2. After detecting the content of free TDI monomer in the rigid prepolymer, according to the molar ratio of diethylamine (CAS: 109-89-7) to free TDI monomer of 1.1:1, diethylamine was premixed and diluted in anhydrous butyl acetate to prepare a capping solution. The capping solution was then added dropwise to the rigid prepolymer, and the adding time was controlled at 30 min. After the addition was completed, the mixture was kept warm and stirred for 45 min to obtain the capping intermediate.

[0073] D3. Based on a mass ratio of dibutyltin disilicate to the end-capping intermediate of 0.05:100, dibutyltin disilicate was dissolved in anhydrous butyl acetate to prepare a catalytic solution. The catalytic solution was then added to the end-capping intermediate and stirred to prepare a catalytic system. Based on a mass ratio of the end-capping intermediate to hexamethylene diisocyanate trimer (purchased from Wuhan Kemike Biomedical Technology Co., Ltd., HDI-100) of 1:0.15, the HDI trimer was added dropwise to the catalytic system. After the addition was completed, the system was kept at 75°C for 3 hours. After removing volatile impurities in a vacuum environment at 75°C, anhydrous butyl acetate was added to adjust the solid content to 76 wt% to obtain a composite curing agent.

[0074] Comparative Example 2

[0075] Comparative Example 2 provides a method for preparing a composite curing agent, comprising the following steps:

[0076] D1. Isophorone diisocyanate (IPDI, purchased from Wanhua Chemical Group Co., Ltd.) and polycaprolactone diol (PCL-1000, purchased from Greenlink (Jining) Chemical Technology Co., Ltd.) were dried and dehydrated separately in a vacuum environment at 50°C. Under a nitrogen atmosphere, PCL-1000 was added to anhydrous butyl acetate and stirred and mixed in a water bath at 40°C. Then, IPDI was added dropwise to the reaction system, controlling the total amount of IPDI added to the molar ratio of PCL-1000 to 2.1:1. After the addition was completed, the temperature was raised to 75°C and the reaction was stirred at 300 rpm. During the reaction, samples were taken every 30 minutes to titrate and measure the isocyanate content. When the deviation of the isocyanate content from the theoretical value was less than or equal to 5%, the reaction was considered complete and the reaction time was recorded as 4 hours and 10 minutes. After the reaction was completed, a flexible prepolymer was obtained.

[0077] D2. Trimethylolpropane (CAS: 77-99-6, TMP) was melted at 70°C in a vacuum environment to obtain molten trimethylolpropane. Under a nitrogen atmosphere, molten TMP was added dropwise to toluene diisocyanate (purchased from Shandong Jiaxu Chemical Co., Ltd., TDI) preheated at 70°C. During the addition process, the mixture was stirred at 600 rpm, and the total amount added was controlled so that the molar ratio of isocyanate in TDI to hydroxyl groups in TMP was 3.1:1. After the addition was completed, the mixture was kept at 70°C and stirred for 2 hours to obtain a rigid prepolymer.

[0078] D3. In a nitrogen atmosphere at 70°C, the rigid prepolymer was added dropwise to the flexible prepolymer. During the addition process, the stirring speed was maintained at 300 rpm, and the total addition time was controlled to be 30 min. After the addition was completed, the molar ratio of isocyanate groups in the flexible prepolymer to that in the rigid prepolymer was 1:3. The composite precursor was obtained after stirring at 300 rpm for 2 h at 70°C.

[0079] D4. After cooling the composite precursor to 50℃ and holding it at that temperature for 1 hour, the content of free TDI monomer in the composite precursor was detected. Based on the molar ratio of diethylamine (CAS: 109-89-7) to free TDI monomer of 1.1:1, diethylamine was pre-mixed and diluted in anhydrous butyl acetate to prepare a capping solution. The capping solution was then added dropwise to the composite precursor, with the addition time controlled at 30 min. After the addition was completed, the mixture was kept at that temperature and stirred for 45 min. After removing volatile impurities in a vacuum environment at 75℃, anhydrous butyl acetate was added to adjust the solid content to 76 wt%, thus obtaining the composite curing agent.

[0080] Comparative Example 3

[0081] Comparative Example 3 provides a method for preparing a composite curing agent, comprising the following steps:

[0082] D1. Isophorone diisocyanate (IPDI, purchased from Wanhua Chemical Group Co., Ltd.) and polycaprolactone diol (PCL-1000, purchased from Greenlink (Jining) Chemical Technology Co., Ltd.) were dried and dehydrated separately in a vacuum environment at 50°C. Under a nitrogen atmosphere, PCL-1000 was added to anhydrous butyl acetate and stirred and mixed in a water bath at 40°C. Then, IPDI was added dropwise to the reaction system, controlling the total amount of IPDI added to the molar ratio of PCL-1000 to 2.1:1. After the addition was completed, the temperature was raised to 75°C and the reaction was stirred at 300 rpm. During the reaction, samples were taken every 30 minutes to titrate and measure the isocyanate content. When the deviation of the isocyanate content from the theoretical value was less than or equal to 5%, the reaction was considered complete and the reaction time was recorded as 4 hours and 10 minutes. After the reaction was completed, a flexible prepolymer was obtained.

[0083] D2. Trimethylolpropane (CAS: 77-99-6, TMP) was melted at 70°C in a vacuum environment to obtain molten trimethylolpropane. Under a nitrogen atmosphere, molten TMP was added dropwise to toluene diisocyanate (purchased from Shandong Jiaxu Chemical Co., Ltd., TDI) preheated at 70°C. During the addition process, the mixture was stirred at 600 rpm, and the total amount added was controlled so that the molar ratio of isocyanate in TDI to hydroxyl groups in TMP was 3.1:1. After the addition was completed, the mixture was kept at 70°C and stirred for 2 hours to obtain a rigid prepolymer.

[0084] D3. After detecting the content of free TDI monomer in the flexible prepolymer, a diethylamine anhydrous butyl acetate solution (the molar ratio of diethylamine to free TDI monomer is 1.1:1) is added dropwise to the flexible prepolymer, controlling the adding time to 30 min. After the addition is completed, the mixture is kept at a constant temperature and stirred for 45 min to obtain the end-capped flexible prepolymer. After detecting the content of free TDI monomer in the rigid prepolymer, a diethylamine anhydrous butyl acetate solution (the molar ratio of diethylamine to free TDI monomer is 1.1:1) is added dropwise to the rigid prepolymer, controlling the adding time to 30 min. After the addition is completed, the mixture is kept at a constant temperature and stirred for 45 min to obtain the end-capped rigid prepolymer.

[0085] D4. Based on the molar ratio of isocyanate groups in the flexible prepolymer to isocyanate groups in the rigid prepolymer being 1:3, the end-capped rigid prepolymer and the end-capped flexible prepolymer are mixed, and volatile impurities are removed in a vacuum environment at 75°C. Then, anhydrous butyl acetate is added to adjust the solid content to 75wt% to obtain a composite curing agent.

[0086] Performance testing

[0087] The isocyanate (-NCO) group content of the composite curing agents prepared in Example 1 and Comparative Examples 1 to 3 was determined according to the method described in GB / T 12009.4, and the results are shown in Table 1 below. The free TDI monomer content of the composite curing agents prepared in Example 1 and Comparative Examples 1 to 3 was determined according to the method described in GB / T 18446-2009, and the results are shown in Table 1 below.

[0088] The composite curing agents from Examples 1 and 1 to 3 were applied to two-component polyurethane coatings to test the performance of the polyurethane coatings. The two-component polyurethane coatings comprised hydroxyl acrylic resin and a composite curing agent. The hydroxyl acrylic resin and the composite curing agent were uniformly mixed at a mass ratio of 1:0.5 and then coated onto a stainless steel plate for curing to obtain a polyurethane coating. The surface drying time of the coating was determined according to the method described in GB / T 1728-2020; the hardness of the coating was determined according to the method described in GB / T 1730-2007; the adhesion of the coating was determined according to the method described in GB / T 9286-2021; and the weather resistance rating of the coating was determined according to the methods described in GB / T 1865-2009 and GB / T 1766-2008. The test results are shown in Table 1 below.

[0089] Table 1 Performance Testing

[0090] Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 NCO / % 13.32 15.74 11.28 10.41 Free TDI monomer / % 0.18 0.28 0.20 0.21 Surface drying time / min 18.4 15.6 23.2 23.5 hardness 2H 2H H H Adhesion / Grade 0 2 1 2 Overall weather resistance / grade 0 2 2 2

[0091] While embodiments of the present invention have been described in detail above, it will be apparent to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations fall within the scope and spirit of the invention as set forth in the claims. Furthermore, the invention described herein may have other embodiments and can be implemented or carried out in various ways.

Claims

1. A method for preparing a composite curing agent, characterized in that, include: A flexible prepolymer was prepared by reacting isophorone diisocyanate with polyester polyol in an anhydrous solvent at 70℃-80℃; a rigid prepolymer was prepared by reacting toluene diisocyanate with molten trimethylolpropane at 60℃-80℃; a composite precursor was prepared by reacting the flexible prepolymer with the rigid prepolymer at 65℃-75℃; a capped intermediate was prepared by end-capping the composite precursor at 45℃-55℃; and a composite curing agent was prepared by grafting the capped intermediate with hexamethylene diisocyanate trimer at 70℃-80℃ under the action of a catalyst.

2. The preparation method according to claim 1, characterized in that: The anhydrous solvent includes anhydrous butyl acetate; and / or, the molar ratio of isophorone diisocyanate to polyester polyol is (1.8-2.3):1; and / or, isophorone diisocyanate and polyester polyol are mixed and reacted in an anhydrous protective atmosphere; and / or, isophorone diisocyanate and polyester polyol are pre-dehydrated and dried; and / or, a flexible prepolymer is obtained after mixing and reacting for 3-5 hours; and / or, the polyester polyol includes polycaprolactone diol.

3. The preparation method according to claim 1, characterized in that: When isophorone diisocyanate is mixed with polyester polyol, the reaction endpoint is considered to be reached when the deviation of the isocyanate content in the system from the theoretical value is less than or equal to 5%; and / or, the molar ratio of isocyanate in toluene diisocyanate to hydroxyl group in trimethylolpropane is (3-3.2):1; and / or, toluene diisocyanate and trimethylolpropane are dehydrated and dried; and / or, molten trimethylolpropane is added dropwise to toluene diisocyanate.

4. The preparation method according to claim 1, characterized in that: Toluene diisocyanate includes one of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and / or, toluene diisocyanate is mixed with molten trimethylolpropane and reacted for 1-3 hours; and / or, toluene diisocyanate is stirred with molten trimethylolpropane and reacted at a speed of 500-800 rpm; and / or, a rigid prepolymer is added dropwise to a flexible prepolymer.

5. The preparation method according to claim 1, characterized in that: The molar ratio of isocyanate groups in the flexible prepolymer to isocyanate groups in the rigid prepolymer is 1:(2-4); and / or, the flexible prepolymer and the rigid prepolymer are mixed and reacted at a speed of 300rpm-500rpm; and / or, the flexible prepolymer and the rigid prepolymer are mixed and reacted for 1h-3h.

6. The preparation method according to claim 1, characterized in that, Before capping the composite precursor, the content of free toluene diisocyanate monomer in the composite precursor was detected. The composite precursor and capping agent were then mixed and the capping reaction was carried out at 45℃-55℃.

7. The preparation method according to claim 6, characterized in that: The capping agent includes diethylamine; and / or, the molar ratio of the capping agent to the free toluene diisocyanate monomer in the composite precursor is (1-1.2):1; and / or, the capping agent is pre-diluted to prepare a capping solution, and then the capping solution is added dropwise to the composite precursor; and / or, the composite precursor and the capping agent are mixed and reacted for 30-60 minutes.

8. The preparation method according to claim 1, characterized in that: The catalyst includes one of dibutyltin dilaurate and stannous octoate; and / or, the mass ratio of the catalyst to the end-capping intermediate is (0.01-0.1):100; and / or, the catalyst is prepared into a catalytic solution beforehand, and then the catalytic solution is mixed with the end-capping intermediate to obtain the catalytic system; and / or, the end-capping intermediate is mixed with hexamethylene diisocyanate trimer and reacted for 2-4 hours; and / or, the mass ratio of the end-capping intermediate to hexamethylene diisocyanate trimer is 1:(0.12-0.18); and / or, after mixing and grafting, volatile impurities are removed in a vacuum environment at 70℃-80℃ and the solid content is adjusted.

9. A composite curing agent prepared by the preparation method according to any one of claims 1 to 8, characterized in that, The content of free toluene diisocyanate monomer in the composite curing agent is less than or equal to 0.2 wt%, and the solid content is 75 wt%-77 wt%.

10. The application of a composite curing agent prepared by any one of claims 1 to 8 in polyurethane with low TDI content.

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