Polymerized MDI and preparation method thereof

Abstract

The invention provides polymeric MDI and a preparation method thereof, and the polymeric MDI comprises the following components in percentage by mass: 20-65% of methylene diphenyl diisocyanate, 15-40% of dimethylene triphenyl triisocyanate, 5-20% of trimethylene tetraphenyl tetraisocyanate, 1-8% of tetramethylene pentaphenyl pentaisocyanate, 1-5% of a catalyst, 1-5% of an antioxidant, 1-5% of a coupling agent, 1-5% of a coupling agent, 1-5% of a coupling agent, 1-5% of a coupling agent, and the balance of water. 8-30% of polymethylene polyphenyl polyisocyanate and MDI high polymer, and 1-2% of uretdione; the polymethylene polyphenyl polyisocyanate is selected from at least one of isocyanates containing more than 6 phenyl groups. According to the polymeric MDI provided by the invention, the content of uretdione is only 1-2% under the condition that key indexes such as the content of isocyanate, the viscosity and the NCO mass content are normal, the content of uretdione can be further controlled to be 1.41-1.92%, and the shelf life can be prolonged to more than one year at normal temperature.

Description

Technical Field

[0001] The present invention belongs to the technical field of isocyanate production and manufacturing, and specifically relates to a polymeric MDI and a preparation method thereof. Background Art

[0002] Polymeric MDI, short for polyphenyl polymethylene polyisocyanate, is a widely used isocyanate and an important raw material for preparing polyurethane materials, and is widely used in fields such as refrigerators, freezers, wood-based panels, building insulation, adhesives, etc.

[0003] Currently, polymeric MDI is mainly prepared by the phosgene method at home and abroad. The process steps are briefly described as follows: Aniline and formaldehyde undergo a condensation reaction under the catalysis of an acid catalyst to form a mixture of diphenylmethylenediamine and polyphenyl polymethylene polyamine, which is called crude MDA. After the crude MDA is mixed with an inert solvent, it is then subjected to a phosgenation reaction with phosgene to obtain a phosgenation reaction solution. The phosgenation reaction solution is subjected to phosgene removal and solvent removal to obtain a crude isocyanate product, called crude MDI. Crude MDI is a mixture of bicyclic and polycyclic isocyanates. Crude MDI distills out bicyclic MDI in a certain proportion through distillation or rectification, and the bottom product is polymeric MDI.

[0004] Polymeric MDI is a liquid at room temperature, and the normal storage temperature is 20 - 30°C. During storage, it will slowly undergo a self-polymerization reaction to form uretidione. This reaction is a reaction in which the NCO groups at each end of two MDI molecules combine to form a -N-C-N-C- four-membered ring. The self-polymerization reaction is as follows: Uretidione has a certain solubility in polymeric MDI. As the storage time prolongs, the content of uretidione gradually increases. When it reaches the saturation precipitation point, uretidione will precipitate in the form of a solid, resulting in the turbidity of polymeric MDI and making it unable to be used continuously.

[0005] The formation reaction of uretidione not only occurs during storage, but also occurs at a faster rate under the high-temperature conditions of the bottom of the tower during distillation or rectification. Therefore, polymeric MDI just produced from the device inevitably contains a certain amount of uretidione. Therefore, in order to make polymeric MDI have better storage stability and a longer shelf life, reducing the initial value of uretidione in polymeric MDI is a key issue.

[0006] Patent WO2022194621A1 discloses that adding heterocyclic compounds containing atoms such as N, Cl, S, etc. to 4,4-MDI can slow down the growth rate of uretidione during storage, but the effect is relatively limited from the data, and adding such compounds will have an impact on the color, activity, etc. of the product.

[0007] In the polymeric MDI produced by the prior art, the content of uretidione is basically about 2.2 - 2.4%, and when stored at room temperature, the growth rate of uretidione is 0.06% per month. Since its precipitation point is about 2.5%, the shelf life is generally within 6 months. Therefore, reducing the initial value of uretidione in the polymeric MDI at the time of just production can effectively extend its storage shelf life. In the currently publicly available polymeric MDI production technologies at home and abroad, there is no public report on the control value of the uretidione content in the polymeric MDI and the specific control process and method.

[0008] In summary, it is urgent to solve the problem of high initial content of uretidione in the polymeric MDI to extend its shelf life. Summary of the Invention

[0009] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a polymeric MDI with improved storage stability and its preparation method, by reducing the initial content of uretidione in the polymeric MDI to extend its shelf life, so that the storage time at room temperature can reach more than 6 months, even more than one year.

[0010] To achieve this purpose, the present invention adopts the following technical solutions:

[0011] In the first aspect, the present invention provides a polymeric MDI, which comprises the following components in mass percentage: methylene diphenyl diisocyanate 20 - 65%, dimethylene triphenyl triisocyanate 15 - 40%, trimethylene tetraphenyl tetraisocyanate 5 - 20%, tetramethylene pentaphenyl pentaisocyanate 1 - 8%, polymethylene polyphenyl polyisocyanate and MDI polymer 8 - 30%, uretidione 1 - 2%; the polymethylene polyphenyl polyisocyanate is selected from at least one of the isocyanates containing more than 6 benzene rings.

[0012] The mass percentage of methylene diphenyl diisocyanate in the polymeric MDI is 20 - 65%, for example, it can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60% or 65%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0013] The mass percentage of dimethylene triphenyl triisocyanate in the polymeric MDI is 15 - 40%, for example, it can be 15%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38% or 40%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0014] The mass percentage content of trimethylene tetraphenyl tetraisocyanate in the polymeric MDI is 5-20%, for example, it can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0015] The mass percentage content of tetramethylene pentaphenyl pentaisocyanate in the polymeric MDI is 1-8%, for example, it can be 1%, 2%, 3%, 4%, 5%, 6%, 7% or 8%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0016] The mass percentage content of polymethylene polyphenyl polyisocyanate and MDI polymer in the polymeric MDI is 8-30%, for example, it can be 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28% or 30%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0017] The mass percentage content of uretidione in the polymeric MDI is 1-2%, for example, it can be 1%, 1.1%, 1.2%, 1.4%, 1.6%, 1.8% or 2%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0018] For the polymeric MDI provided by the present invention, when the key indicators such as its viscosity, NCO mass content and different MDI contents are normal, by reducing the initial uretidione content of the polymeric MDI, its storage time can be extended, and the shelf life can be extended from 6 months to more than 1 year.

[0019] The following are the preferred technical solutions of the present invention, but do not limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the purpose and beneficial effects of the present invention can be better achieved and realized.

[0020] As a preferred technical solution, the viscosity of the polymeric MDI at 25°C is 50-1000 cP, for example, it can be 50 cP, 100 cP, 150 cP, 200 cP, 300 cP, 400 cP, 500 cP, 600 cP, 700 cP, 800 cP, 900 cP or 1000 cP, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0021] Preferably, the mass content of NCO in the polymeric MDI is 29-32%, for example, it can be 29%, 29.2%, 29.5%, 29.8%, 30%, 30.2%, 30.5%, 30.8%, 31%, 31.2%, 31.5%, 31.8% or 32%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0022] Preferably, the mass percentage content of uretidione in the polymeric MDI is 1-1.6%, for example, it can be 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or 1.6%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0023] Preferably, the number average molecular weight of the MDI polymer is 450-10000, for example, it can be 450, 500, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000 or 10000, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0024] The MDI polymer is formed by self-polymerization of at least two MDI molecules. Exemplarily, the MDI polymer includes the following compounds:

[0025]

[0026] In a second aspect, the present invention provides a method for preparing the polymeric MDI as described in the first aspect. The preparation method includes: distilling out part of methylene diphenyl diisocyanate from the crude MDI by rectification or distillation operations according to a separation ratio of (0.05-1.5):1, and then cooling the bottoms product to obtain the polymeric MDI; the separation ratio is the ratio of the mass of the distilled methylene diphenyl diisocyanate to the mass of the bottoms product.

[0027] The rectification or distillation operations in the present invention are carried out in chemical equipment. Exemplarily, the chemical equipment includes, but is not limited to, falling film evaporators, scraping evaporators, molecular distillers, distillation columns, rectification columns, etc., and the rectification column includes a dividing wall rectification column.

[0028] The crude MDI in the present invention is a crude isocyanate product, and the crude MDI includes first crude MDI or second crude MDI; the preparation method of the first crude MDI includes the phosgene method, and the phosgene method includes the following steps:

[0029] (1) Aniline and formaldehyde react under the catalysis of hydrochloric acid to obtain crude MDA; the mass ratio of aniline, formaldehyde and hydrochloric acid is 1:(0.3 - 0.7):(0.05 - 0.5); the temperature of the reaction is 30 - 150 °C; the time of the reaction is 1 - 6 h;

[0030] (2) The crude MDA, inert solvent and phosgene are mixed and then subjected to a phosgenation reaction to obtain a phosgenation reaction solution; the temperature of the phosgenation reaction is 50 - 200 °C; the time of the phosgenation reaction is 1 - 60 min; the inert solvent includes any one or a combination of at least two of chlorobenzene, o-dichlorobenzene, toluene or xylene; the mass ratio of the crude MDA, inert solvent to phosgene is 1:(2 - 4):(2 - 6);

[0031] (3) After the phosgenation reaction solution is subjected to phosgene removal and solvent removal, the first crude MDI is obtained; the specific operation of phosgene removal is: removing the unreacted phosgene in the phosgenation reaction solution by a heat treatment method, the temperature of the heat treatment is 150 - 170 °C, preferably 160 - 165 °C; the time of the heat treatment is 10 - 30 min, preferably 25 - 28 min; the phosgene content in the phosgenation reaction solution after the heat treatment is reduced to 80 - 200 ppm; the specific operation of solvent removal is: removing the solvent in the phosgenation reaction solution after the heat treatment by a vacuum distillation method, the pressure of the vacuum distillation is 1.2 - 40 kPaA, preferably 1.5 - 2 kPaA; the temperature of the vacuum distillation is 180 - 210 °C, preferably 195 - 205 °C; the time of the vacuum distillation is 10 - 20 min, preferably 12 - 18 min.

[0032] Preferably, the second crude MDI is obtained after the first crude MDI is stored.

[0033] Preferably, the storage time is 0.1 - 6 months, for example, it can be 0.1 month, 0.5 month, 1 month, 1.5 months, 2 months, 2.5 months, 3 months, 3.5 months, 4 months, 4.5 months, 5 months, 5.5 months or 6 months, as well as the specific point values between the above point values. Due to space limitations and for the sake of brevity, the specific point values included in the scope of the present invention are not exhaustively listed herein.

[0034] Preferably, the storage temperature is 10 - 50 °C, for example, it can be 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C or 50 °C, as well as the specific point values between the above point values. Due to space limitations and for the sake of brevity, the specific point values included in the scope of the present invention are not exhaustively listed herein.

[0035] Preferably, the crude MDI comprises the following components in terms of mass percentage: 40-75% of methylene diphenyl diisocyanate, 10-30% of dimethylene triphenyl triisocyanate, 3-15% of trimethylene tetraphenyl tetraisocyanate, 1-6% of tetramethylene pentaphenyl pentaisocyanate, 6-25% of polymethylene polyphenyl polyisocyanate and MDI polymer, and 1-5% of uretidione; the polymethylene polyphenyl polyisocyanate is selected from at least one of the isocyanates containing more than 6 benzene rings.

[0036] The mass percentage of methylene diphenyl diisocyanate in the crude MDI is 40-75%, for example, it can be 40%, 45%, 50%, 55%, 60%, 65%, 70% or 75%, and the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0037] The mass percentage of dimethylene triphenyl triisocyanate is 10-30%, for example, it can be 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28% or 30%, and the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0038] The mass percentage of trimethylene tetraphenyl tetraisocyanate is 3-15%, for example, it can be 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%, and the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0039] The mass percentage of tetramethylene pentaphenyl pentaisocyanate is 1-6%, for example, it can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5% or 6%, and the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0040] The mass percentage of polymethylene polyphenyl polyisocyanate and MDI polymer is 6-25%, for example, it can be 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22% or 25%, and the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0041] The mass percentage content of the uredione is 1-5%, for example, it can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0042] Preferably, the number-average molecular weight of the MDI polymer is 450-10000.

[0043] Preferably, the bottom temperature of the rectification or distillation operation is 190-230°C, for example, it can be 190°C, 196°C, 200°C, 205°C, 210°C, 215°C, 219°C, 225°C or 230°C, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0044] More preferably, the bottom temperature of the rectification or distillation operation is 195-220°C, for example, it can be 195°C, 198°C, 200°C, 202°C, 205°C, 208°C, 210°C, 215°C, 218°C or 220°C, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0045] Since the reaction of two MDI molecules self-polymerizing to form uredione is a reversible reaction, as the temperature increases, the formation rate of uredione increases significantly, but eventually reaches an equilibrium value. And this equilibrium value gradually increases below 180°C and gradually decreases above 180°C. That is, when the temperature exceeds 180°C, the reaction proceeds in the reverse direction, that is, uredione decomposes to form MDI. Therefore, the higher the temperature, the greater the degree of reverse equilibrium, that is, the lower the content of uredione.

[0046] In the phosgenation reaction and solvent removal stage, due to the high temperature experienced, the crude MDI already contains a relatively high amount of uredione, generally between 1.5-2.5%. After entering the distillation or rectification device, based on the above decomposition equilibrium theory of uredione, the higher the bottom temperature, the more beneficial it is to reduce the content of uredione. However, at higher temperatures, MDI will undergo another self-polymerization reaction to form carbodiimide and carbon dioxide. This reaction will not only reduce the quality of the polymeric MDI, but also bring safety risks such as solidification of the kettle and overpressure explosion. Therefore, the bottom temperature is not the higher the better.

[0047] Preferably, the top pressure of the rectification or distillation operation is 20 - 1000 Pa (absolute pressure), for example, it can be 20 Pa, 50 Pa, 100 Pa, 150 Pa, 200 Pa, 300 Pa, 400 Pa, 500 Pa, 600 Pa, 700 Pa, 800 Pa, 900 Pa or 1000 Pa, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0048] Preferably, the top pressure of the rectification or distillation operation is 300 - 800 Pa (absolute pressure), for example, it can be 300 Pa, 350 Pa, 400 Pa, 450 Pa, 500 Pa, 550 Pa, 600 Pa, 650 Pa, 700 Pa, 750 Pa or 800 Pa, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0049] Preferably, the residence time of the crude MDI in the column kettle during the rectification or distillation operation is 0.5 - 60 min, for example, it can be 0.5 min, 1 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0050] Preferably, when the mass percentage content of uretidione in the crude MDI exceeds 2%, the column kettle temperature of the rectification or distillation operation is 200 - 220 °C, for example, it can be 200 °C, 202 °C, 204 °C, 206 °C, 208 °C, 210 °C, 212 °C, 214 °C, 216 °C, 218 °C or 220 °C, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0051] Preferably, when the mass percentage content of uretidione in the crude MDI exceeds 2%, the residence time of the crude MDI in the column kettle during the rectification or distillation operation is 20 - 60 min, for example, it can be 20 min, 22 min, 24 min, 26 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0052] The residence time of the crude MDI in the bottom of the column is not the shorter the better, and it needs to be controlled within a suitable range. Since the storage time of the crude MDI in the storage tank is uncertain, when using crude MDI with a longer storage time, the content of uretdione in it is relatively high, and may even exceed 5% or more. At this time, in order to reduce the uretdione content in the polymeric MDI product, the bottom temperature of the column needs to be increased as much as possible, preferably 200 - 220 °C. In addition, the residence time of the bottom of the column needs to be extended so that uretdione has enough time to decompose. The residence time is preferably controlled within 20 - 60 min. Continuing to extend the residence time is beneficial to the decomposition of uretdione, but since the reaction of forming carbodiimide will occur simultaneously, it will lead to an increase in the viscosity of the product and a decrease in the NCO content, which is not conducive to the application of the product.

[0053] Preferably, the reflux ratio in the rectification operation is 0.05 - 1. For example, it can be 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the specific point values included in the scope of the present invention are not exhaustively listed herein.

[0054] Preferably, the temperature of the bottom product is cooled to below 90 °C (such as 10 °C, 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C or 90 °C) within 120 s (such as 10 s, 20 s, 30 s, 40 s, 50 s, 60 s, 70 s, 80 s, 90 s, 100 s, 110 s or 120 s) through the cooling, preferably cooled to below 85 °C (such as 10 °C, 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C or 85 °C) within 60 s (such as 10 s, 20 s, 30 s, 40 s, 50 s or 60 s), and further preferably cooled to below 80 °C (such as 10 °C, 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C or 80 °C) within 20 s (such as 2 s, 4 s, 6 s, 8 s, 10 s, 12 s, 14 s, 16 s, 18 s or 20 s).

[0055] The bottom product needs to be cooled to obtain the polymeric MDI. The cooling process is also an important stage for the increase in the uretdione content. In the temperature range of 120 - 170 °C, the growth rate of uretdione is fast and the equilibrium point is high. The shorter the residence time in this interval, the smaller the increase in uretdione. Therefore, it is necessary to control the cooling duration to reduce the uretdione content in the polymeric MDI.

[0056] Preferably, the cooling method includes at least one of direct mixing heat exchange or wall heat exchange.

[0057] Preferably, the cold fluid for the direct mixing heat exchange is low-temperature polymeric MDI, and the composition of the low-temperature polymeric MDI is the same as that of the polymeric MDI.

[0058] Preferably, the temperature of the low-temperature polymeric MDI is 45 - 60 °C, for example, it can be 45 °C, 46 °C, 48 °C, 50 °C, 51 °C, 53 °C, 54 °C, 56 °C, 57 °C, 59 °C or 60 °C, as well as the specific point values between the above point values. Due to space limitations and for the sake of brevity, the specific point values included in the scope of the present invention are not exhaustively listed herein.

[0059] Preferably, in the direct mixing heat exchange, the mass flow ratio of the low-temperature polymeric MDI to the bottoms product is (3 - 10):1, for example, it can be 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1 or 10:1, etc.

[0060] More preferably, in the direct mixing heat exchange, the mass flow ratio of the low-temperature polymeric MDI to the bottoms product is (5 - 8):1, for example, it can be 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1 or 8:1, etc.

[0061] In order to achieve the purpose of rapid cooling, by controlling the temperature of the low-temperature polymeric MDI and the mass flow ratio of the low-temperature polymeric MDI to the bottoms product, it can be ensured that the bottoms product drops from above 190 °C to below 90 °C within 1 - 5 s, and basically there is no risk of increasing the biuret content.

[0062] Preferably, the heat exchange equipment for the shell-and-tube heat exchange includes any one of a plate heat exchanger, a spiral plate heat exchanger, or a shell-and-tube heat exchanger.

[0063] Without additional equipment or additives, the present invention can control the biuret content in the polymeric MDI to 1 - 2% by simply adjusting the process parameters of the rectification or distillation operation, and further, the biuret content can be controlled to 1.41 - 1.92%.

[0064] Preferably, the preparation method specifically includes: a part of methylene diphenyl diisocyanate is distilled out from the crude MDI through rectification or distillation operation according to a separation ratio of (0.05 - 1.5):1, and then the bottoms product is cooled to obtain the polymeric MDI.

[0065] The crude MDI includes the first crude MDI or the second crude MDI.

[0066] The preparation method of the first crude MDI includes the following steps:

[0067] (1) Aniline and formaldehyde react under the catalysis of hydrochloric acid to obtain crude MDA; the mass ratio of aniline, formaldehyde and hydrochloric acid is 1:(0.3 - 0.7):(0.05 - 0.5); the temperature of the reaction is 30 - 150 °C; the time of the reaction is 1 - 6 h;

[0068] (2) The crude MDA, an inert solvent and phosgene are mixed and then subjected to a phosgenation reaction to obtain a phosgenation reaction solution; the temperature of the phosgenation reaction is 50 - 200 °C; the time of the phosgenation reaction is 1 - 60 min; the mass ratio of the crude MDA, the inert solvent to phosgene is 1:(2 - 4):(2 - 6);

[0069] (3) After the phosgenation reaction solution is subjected to phosgene removal and solvent removal, the first crude MDI is obtained.

[0070] The second crude MDI is obtained after the first crude MDI is stored.

[0071] The storage time is 0.1 - 6 months; the storage temperature is 10 - 50 °C.

[0072] The separation ratio is the ratio of the mass of the methylene diphenyl diisocyanate distilled out to the mass of the bottoms product.

[0073] The bottoms temperature of the rectification or distillation operation is 190 - 230 °C, more preferably 195 - 220 °C.

[0074] The top pressure of the rectification or distillation operation is 20 - 1000 Pa (absolute pressure), preferably 300 - 800 Pa (absolute pressure).

[0075] The residence time of the crude MDI at the bottom of the column in the rectification or distillation operation is 0.5 - 60 min.

[0076] When the mass percentage content of uretidione in the crude MDI exceeds 2%, the bottoms temperature of the rectification or distillation operation is 200 - 220 °C, and the residence time of the crude MDI at the bottom of the column in the rectification or distillation operation is 20 - 60 min.

[0077] The reflux ratio in the rectification operation is 0.05 - 1.

[0078] The temperature of the bottoms product is cooled to below 90 °C within 120 s by the cooling, preferably cooled to below 85 °C within 60 s, and more preferably cooled to below 80 °C within 20 s.

[0079] The cooling method includes at least one of direct mixing heat exchange or wall - type heat exchange.

[0080] The cold fluid of the direct mixing heat exchange is low-temperature polymeric MDI, and the composition of the low-temperature polymeric MDI is the same as that of the polymeric MDI.

[0081] The temperature of the low-temperature polymeric MDI is 45 - 60 °C.

[0082] The mass flow ratio of the low-temperature polymeric MDI to the bottoms product is (3 - 10):1, preferably (5 - 8):1.

[0083] Compared with the prior art, the present invention has the following beneficial effects:

[0084] When the polymeric MDI provided by the present invention maintains normal key indicators such as various isocyanate contents, viscosities, and NCO mass contents, the biuret content is as low as 1 - 2%, and the biuret content can be further controlled within 1.41 - 1.92%, and the shelf life can be extended to more than 1 year. Description of the Drawings

[0085] Figure 1 It is a process flow diagram of the preparation of the polymeric MDI provided in Example 2. Detailed Embodiments

[0086] The technical solution of the present invention will be further described below in conjunction with the drawings and through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations to the present invention.

[0087] The sources of some components in the following examples and comparative examples are as follows:

[0088] Preparation Example 1

[0089] A crude MDI and its preparation method, the preparation method comprising the following steps:

[0090] (1) Add aniline, formaldehyde, and hydrochloric acid to the reaction kettle according to a mass ratio of 1:0.45:0.3, with a reaction temperature of 60 °C, control the reaction time to 2.3 h, and obtain crude MDA through a condensation reaction;

[0091] (2) Mix the crude MDA, chlorobenzene, and phosgene in a mixer according to a mass ratio of 1:2.5:3.2, and add them to the reaction kettle, and carry out a phosgenation reaction at 90 °C for 40 min to obtain a phosgenation reaction solution;

[0092] (3) The photochemical reaction liquid enters a phosgene removal tower for heat treatment to remove phosgene. The bottom temperature of the phosgene removal tower is set at 161 °C, and the residence time of the photochemical reaction liquid at the bottom of the tower is 26 min. After the heat treatment, the phosgene content in the photochemical reaction liquid is 150 ppm. Then, the photochemical reaction liquid after phosgene removal is subjected to solvent removal at 2 kPaA, a temperature of 195 °C, and a time of 18 min to obtain the crude MDI.

[0093] The crude MDI includes the following components by mass percentage: methylene diphenyl diisocyanate 60.8%, dimethylene triphenyl triisocyanate 13.1%, trimethylene tetraphenyl tetraisocyanate 6.8%, tetramethylene pentaphenyl pentaisocyanate 3.2%, polymethylene polyphenyl polyisocyanate and MDI polymer 13.9%, and uretidione 2.2%.

[0094] Preparation Example 2

[0095] A crude MDI and a preparation method thereof, the preparation method comprising the following steps:

[0096] (1) Aniline, formaldehyde, and hydrochloric acid are added to a reaction kettle according to a mass ratio of 1:0.5:0.32, the reaction temperature is 70 °C, the reaction time is controlled for 2 h, and after a condensation reaction, crude MDA is obtained.

[0097] (2) The crude MDA, chlorobenzene, and phosgene are mixed in a mixer according to a mass ratio of 1:2.9:3.7 and added to a reaction kettle, and a photochemical reaction is carried out at 100 °C for 35 min to obtain a photochemical reaction liquid.

[0098] (3) The photochemical reaction liquid enters a phosgene removal tower for heat treatment to remove phosgene. The bottom temperature of the phosgene removal tower is 163 °C, and the residence time of the photochemical reaction liquid at the bottom of the tower is 30 min. After the heat treatment, the phosgene content in the photochemical reaction liquid is 120 ppm. Then, the photochemical reaction liquid after phosgene removal is subjected to solvent removal at 1.5 kPaA, a temperature of 199 °C, and a time of 16 min to obtain a first crude MDI. After the first crude MDI is stored at 45 °C for 3 months, the crude MDI is obtained.

[0099] The crude MDI includes the following components by mass percentage: methylene diphenyl diisocyanate 55.8%, dimethylene triphenyl triisocyanate 13.6%, trimethylene tetraphenyl tetraisocyanate 7.2%, tetramethylene pentaphenyl pentaisocyanate 4.1%, polymethylene polyphenyl polyisocyanate and MDI polymer 15.4%, and uretidione 3.9%.

[0100] Preparation Example 3

[0101] A crude MDI and its preparation method, which is different from Preparation Example 1 in that the phosgenated reaction liquid in step (3) enters a phosgene removal tower for heat treatment to remove phosgene. The bottom temperature of the phosgene removal tower is 160 °C, the residence time of the phosgenated reaction liquid in the bottom of the tower is 15 min, and after the heat treatment, the phosgene content of the phosgenated reaction liquid is 160 ppm; then the phosgenated reaction liquid after phosgene removal is subjected to solvent removal at 2 kPaA, the temperature is 205 °C, and the time is 12 min to obtain the crude MDI.

[0102] The crude MDI includes the following components by mass percentage: methylene diphenyl diisocyanate 60.9%, dimethylene triphenyl triisocyanate 13.2%, trimethylene tetraphenyl tetraisocyanate 7%, tetramethylene pentaphenyl pentaisocyanate 3.2%, polymethylene polyphenyl polyisocyanate and MDI polymer 13.9%, and uretidione 1.8%.

[0103] Example 1

[0104] A polymeric MDI, which includes the following components by mass percentage: methylene diphenyl diisocyanate 45.19%, dimethylene triphenyl triisocyanate 17.76%, trimethylene tetraphenyl tetraisocyanate 9.3%, tetramethylene pentaphenyl pentaisocyanate 4.44%, polymethylene polyphenyl polyisocyanate and MDI polymer 21.9%, and uretidione 1.41%. The viscosity of the polymeric MDI at 25 °C is 199 cP, and the mass content of NCO is 31.1%.

[0105] The preparation process flow of the polymeric MDI is as Figure 1 shown. The preparation method includes: the crude MDI provided in Preparation Example 1 is in a distillation column according to a separation ratio of 0.39:1, and part of the methylene diphenyl diisocyanate is distilled out. The top pressure is 300 Pa (absolute pressure), the bottom temperature of the column is 212 °C, the reflux ratio is 0.3, the residence time of the crude MDI in the bottom of the column is 20 min, and the bottom product is first quickly cooled to 73 °C in 5 s by direct mixing heat exchange. The cold fluid is the polymeric MDI at 50 °C, and the mass flow ratio of the cold fluid to the bottom product is 6:1, and then it is cooled to 50 °C through a common shell-and-tube heat exchanger to obtain the polymeric MDI.

[0106] Example 2

[0107] A polymeric MDI, which comprises the following components in mass percentage: methylene diphenyl diisocyanate 33.54%, dimethylene triphenyl triisocyanate 21.7%, trimethylene tetraphenyl tetraisocyanate 12.5%, tetramethylene pentaphenyl pentaisocyanate 6.9%, polymethylene polyphenyl polyisocyanate and MDI polymer 23.8%, and uretidione 1.56%. The viscosity of the polymeric MDI at 25°C is 590 cP, and the mass content of NCO is 30.6%.

[0108] The preparation method of the polymeric MDI includes: distilling out part of the methylene diphenyl diisocyanate from the crude MDI provided in Preparation Example 2 in a distillation column according to a separation ratio of 0.54:1, wherein the top pressure of the column is 350 Pa (absolute pressure), the bottom temperature of the column is 216°C, the reflux ratio is 0.42, the residence time of the crude MDI in the bottom of the column is 50 min, the bottom product is first rapidly cooled to 75°C in 5 s by direct mixing heat exchange, the cold fluid is the polymeric MDI at 50°C, the mass flow ratio of the cold fluid to the bottom product is 5.6:1, and then it is cooled to 50°C by a common shell-and-tube heat exchanger to obtain the polymeric MDI.

[0109] Example 3

[0110] A polymeric MDI, which comprises the following components in mass percentage: methylene diphenyl diisocyanate 51.25%, dimethylene triphenyl triisocyanate 16.7%, trimethylene tetraphenyl tetraisocyanate 8.7%, tetramethylene pentaphenyl pentaisocyanate 4.1%, polymethylene polyphenyl polyisocyanate and MDI polymer 17.6%, and uretidione 1.65%. The viscosity of the polymeric MDI at 25°C is 140 cP, and the mass content of NCO is 31.7%.

[0111] The preparation method of the polymeric MDI includes: distilling out part of the methylene diphenyl diisocyanate from the crude MDI provided in Preparation Example 1 in a distillation column according to a separation ratio of 0.25:1, wherein the top pressure of the column is 300 Pa (absolute pressure), the bottom temperature of the column is 208°C, the reflux ratio is 0.25, the residence time of the crude MDI in the bottom of the column is 20 min, the bottom product is first rapidly cooled to 80°C in 10 s by direct mixing heat exchange, the cold fluid is the polymeric MDI at 55°C, the mass flow ratio of the cold fluid to the bottom product is 5.1:1, and then it is cooled to 55°C by a common shell-and-tube heat exchanger to obtain the polymeric MDI.

[0112] Example 4

[0113] A polymeric MDI, which comprises the following components by mass percentage: methylene diphenyl diisocyanate 27.54%, dimethylene triphenyl triisocyanate 23.5%, trimethylene tetraphenyl tetraisocyanate 12.4%, tetramethylene pentaphenyl pentaisocyanate 7.2%, polymethylene polyphenyl polyisocyanate and MDI polymer 27.52%, and uretidione 1.84%. The viscosity of the polymeric MDI at 25°C is 750 cP, and the mass content of NCO is 30.4%.

[0114] The preparation method of the polymeric MDI includes: distilling out part of the methylene diphenyl diisocyanate from the crude MDI provided in Preparation Example 2 in a rectifying column according to a separation ratio of 0.67:1, wherein the top pressure of the column is 350 Pa (absolute pressure), the bottom temperature of the column is 210°C, the reflux ratio is 0.45, the residence time of the crude MDI in the bottom of the column is 25 min, the bottom product is first rapidly cooled to 75°C in 5 s by direct mixing heat exchange, the cold fluid is the polymeric MDI at 50°C, the mass flow ratio of the cold fluid to the bottom product is 5.4:1, and then it is cooled to 50°C by a common shell-and-tube heat exchanger to obtain the polymeric MDI.

[0115] Example 5

[0116] A polymeric MDI, which comprises the following components by mass percentage: methylene diphenyl diisocyanate 51.01%, dimethylene triphenyl triisocyanate 16.66%, trimethylene tetraphenyl tetraisocyanate 8.7%, tetramethylene pentaphenyl pentaisocyanate 4.1%, polymethylene polyphenyl polyisocyanate and MDI polymer 17.61%, and uretidione 1.92%. The viscosity of the polymeric MDI at 25°C is 143 cP, and the mass content of NCO is 31.6%.

[0117] The preparation method of the polymeric MDI includes: distilling out part of the methylene diphenyl diisocyanate from the crude MDI provided in Preparation Example 3 in a rectifying column according to a separation ratio of 0.25:1, wherein the top pressure of the column is 300 Pa (absolute pressure), the bottom temperature of the column is 192°C, the reflux ratio is 0.4, the residence time of the crude MDI in the bottom of the column is 20 min, the bottom product is first rapidly cooled to 89°C in 120 s by direct mixing heat exchange, the cold fluid is the polymeric MDI at 55°C, the mass flow ratio of the cold fluid to the bottom product is 3:1, and then it is cooled to 55°C by a common shell-and-tube heat exchanger to obtain the polymeric MDI.

[0118] Example 6

[0119] A polymeric MDI, the polymeric MDI comprising the following components by mass percentage: methylene diphenyl diisocyanate 20.1%, dimethylene triphenyl triisocyanate 26.5%, trimethylene tetraphenyl tetraisocyanate 14.1%, tetramethylene pentaphenyl pentaisocyanate 7.99%, polymethylene polyphenyl polyisocyanate and MDI polymer 29.86%, uretidione 1.45%. The viscosity of the polymeric MDI at 25 °C is 990 cP, and the NCO mass content is 29%.

[0120] The preparation method of the polymeric MDI includes: in the rectification column, part of the methylene diphenyl diisocyanate in the crude MDI provided in Preparation Example 2 is distilled out according to a separation ratio of 0.89:1, wherein the top pressure of the column is 1000 Pa (absolute pressure), the bottom temperature of the column is 220 °C, the reflux ratio is 0.9, the residence time of the crude MDI in the bottom of the column is 60 min, the bottom product of the column is first rapidly cooled to 75 °C in 5 s by direct mixing heat exchange, the cold fluid is the polymeric MDI at 45 °C, the mass flow ratio of the cold fluid to the bottom product of the column is 5.2:1, and then it is cooled to 50 °C by a common shell-and-tube heat exchanger to obtain the polymeric MDI.

[0121] Example 7

[0122] A polymeric MDI, the polymeric MDI comprising the following components by mass percentage: methylene diphenyl diisocyanate 43.5%, dimethylene triphenyl triisocyanate 16.9%, trimethylene tetraphenyl tetraisocyanate 9%, tetramethylene pentaphenyl pentaisocyanate 4.25%, polymethylene polyphenyl polyisocyanate and MDI polymer 24.8%, uretidione 1.55%. The viscosity of the polymeric MDI at 25 °C is 220 cP, and the NCO mass content is 28.8%.

[0123] The difference between the preparation method of the polymeric MDI and that of Example 1 is only that the residence time of the crude MDI in the bottom of the column is 70 min, and other raw materials and process parameters are the same as those in Example 1.

[0124] Comparative Example 1

[0125] A polymeric MDI, the polymeric MDI comprising the following components by mass percentage: methylene diphenyl diisocyanate 45.48%, dimethylene triphenyl triisocyanate 17.3%, trimethylene tetraphenyl tetraisocyanate 9.12%, tetramethylene pentaphenyl pentaisocyanate 4.32%, polymethylene polyphenyl polyisocyanate and MDI polymer 21.48%, uretidione 2.3%. The viscosity of the polymeric MDI at 25 °C is 200 cP, and the NCO mass content is 31%.

[0126] The preparation method of the polymeric MDI is different from that of Example 1 only in that the bottoms product is first cooled by a common shell-and-tube heat exchanger and cooled to 90 °C in 3 minutes, and other raw materials and process parameters are the same as those in Example 1.

[0127] Comparative Example 2

[0128] A polymeric MDI, which comprises the following components in mass percentage: methylene diphenyl diisocyanate 45.5%, dimethylene triphenyl triisocyanate 17.84%, trimethylene tetraphenyl tetraisocyanate 9.45%, tetramethylene pentaphenyl pentaisocyanate 4.41%, polymethylene polyphenyl polyisocyanate and MDI polymer 20.7%, uretidione 2.1%. The viscosity of the polymeric MDI at 25 °C is 200 cP, and the mass content of NCO is 31.1%.

[0129] The preparation method of the polymeric MDI is different from that of Example 1 in that the bottom temperature is 186 °C, and the mass flow ratio of the cold fluid to the bottoms product is 4.9:1, and other raw materials and process parameters are the same as those in Example 1.

[0130] Comparative Example 3

[0131] A polymeric MDI, which comprises the following components in mass percentage: methylene diphenyl diisocyanate 33%, dimethylene triphenyl triisocyanate 21.7%, trimethylene tetraphenyl tetraisocyanate 12.5%, tetramethylene pentaphenyl pentaisocyanate 6.8%, polymethylene polyphenyl polyisocyanate and MDI polymer 23.5%, uretidione 2.5%. The viscosity of the polymeric MDI at 25 °C is 585 cP, and the mass content of NCO is 29.3%.

[0132] The preparation method of the polymeric MDI is different from that of Example 2 only in that the residence time of the crude MDI in the bottom of the column is 15 minutes, the bottom temperature is 195 °C, and the mass flow ratio of the cold fluid to the bottoms product is 4.8:1, and other process parameters are the same as those in Example 2.

[0133] Product physical properties and performance testing:

[0134] (1) Determination of the content of uretidione: It is determined by infrared spectroscopy. The specific steps are as follows: Mix the sample to be tested with dichloromethane according to a weight ratio of 1:30, dissolve it evenly and transfer it into an infrared liquid cell, and perform infrared spectrum scanning. On the spectrum, compare the peak heights at 1780 cm -1 and 1900 cm -1 The obtained ratio is the content of uretidione in the sample to be tested;

[0135] (2) Determination of the content of each isocyanate and MDI polymer in crude MDI or polymeric MDI: High Performance Liquid Chromatography (HPLC), model: Agilent 1200s, chromatographic column Waters SymmetryShield RP18 5 μm. The analysis method is as follows: 0.05 g of the sample to be tested is dissolved in 2 g of dichloromethane, and after adding 2 g of methanol for derivatization, analysis is carried out. The content of each isocyanate and MDI polymer in the sample to be tested is obtained by area normalization of the chromatogram;

[0136] (3) Viscosity determination of polymeric MDI: In accordance with GB / T 12009.3-2016 "Determination of the Viscosity of Polymethylene Polyphenyl Polyisocyanates";

[0137] (4) Determination of the NCO content of polymeric MDI: In accordance with GB / T 12009.4-2016 "Determination of Isocyanate Group Content".

[0138] The polymeric MDI provided in Examples 1-7 and Comparative Examples 1-3 was stored at 20 °C, and its biuret content at different storage durations was tested by the above test methods, and the appearance was observed. The test results are shown in Table 1:

[0139] Table 1

[0140]

[0141]

[0142] It can be seen from Examples 1-6 that the polymeric MDI provided by the present invention remains clear and transparent in appearance after being stored at room temperature for one year, and the shelf life is extended; compared with Example 1, in Example 7, the residence time of the crude MDI in the bottom of the column is extended. The initial biuret content of the produced polymeric MDI has no obvious change, but the polymer content increases and the NCO content decreases, indicating that too long residence time will lead to deeper polymerization of MDI in the bottom of the column, thus deteriorating the performance indicators of the product. From the comparison between Comparative Example 1 and Example 1, it can be seen that the extension of the cooling time of the bottom column product results in a relatively high initial biuret content in the polymeric MDI, leading to a shorter shelf life. From the comparison between Comparative Example 2 and Example 1, it can be seen that when the biuret content in the crude MDI is higher than 2%, the temperature at the bottom of the column is too low, and the decomposition amount of biuret is small, resulting in a relatively high initial biuret content in the polymeric MDI, leading to a shorter shelf life. From the comparison between Comparative Example 3 and Example 2, it can be seen that when the biuret content in the crude MDI is higher than 2%, the temperature at the bottom of the column is too low and the residence time at the bottom of the column is too short, which will result in a relatively high initial biuret content in the polymeric MDI, leading to a shorter shelf life.

[0143] The applicant declares that the detailed process flow of the present invention is illustrated by the above embodiments, 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 present invention's product, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A polymeric MDI, characterized in that, the polymeric MDI comprises the following components by mass percentage: methylene diphenyl diisocyanate 20 - 65%, dimethylene triphenyl triisocyanate 15 - 40%, trimethylene tetraphenyl tetraisocyanate 5 - 20%, tetramethylene pentaphenyl pentaisocyanate 1 - 8%, polymethylene polyphenyl polyisocyanate and MDI polymer 8 - 30%, uretidione 1 - 2%; the polymethylene polyphenyl polyisocyanate is selected from at least one of isocyanates containing more than 6 benzene rings.

2. The polymeric MDI according to claim 1, characterized in that, the viscosity of the polymeric MDI at 25°C is 50 - 1000 cP; preferably, the mass content of NCO in the polymeric MDI is 29 - 32%; preferably, the mass percentage of uretidione in the polymeric MDI is 1 - 1.6%; preferably, the number average molecular weight of the MDI polymer is 450 - 10000.

3. A preparation method of the polymeric MDI according to claim 1 or 2, characterized in that, the preparation method includes: crude MDI is subjected to rectification or distillation operations to distill out part of the methylene diphenyl diisocyanate according to a separation ratio of (0.05 - 1.5):1, and then the bottoms product is cooled to obtain the polymeric MDI; the separation ratio is the ratio of the mass of the distilled methylene diphenyl diisocyanate to the mass of the bottoms product.

4. The preparation method according to claim 3, characterized in that, the crude MDI includes the first crude MDI or the second crude MDI; preferably, the preparation method of the first crude MDI includes the following steps: (1) Aniline and formaldehyde react under the catalysis of hydrochloric acid to obtain crude MDA; the mass ratio of aniline, formaldehyde and hydrochloric acid is 1:(0.3 - 0.7):(0.05 - 0.5); the reaction temperature is 30 - 150°C; the reaction time is 1 - 6 h; (2) The crude MDA, inert solvent and phosgene are mixed and then subjected to a phosgenation reaction to obtain a phosgenation reaction solution; the temperature of the phosgenation reaction is 50 - 200°C; the time of the phosgenation reaction is 1 - 60 min; the mass ratio of the crude MDA, inert solvent to phosgene is 1:(2 - 4):(2 - 6); (3) After the phosgenation reaction solution is subjected to phosgene removal and solvent removal, the first crude MDI is obtained; preferably, the second crude MDI is obtained after the first crude MDI is stored; preferably, the storage time is 0.1 - 6 months; preferably, the storage temperature is 10 - 50°C; preferably, the crude MDI comprises the following components by mass percentage: methylene diphenyl diisocyanate 40 - 75%, dimethylene triphenyl triisocyanate 10 - 30%, trimethylene tetraphenyl tetraisocyanate 3 - 15%, tetramethylene pentaphenyl pentaisocyanate 1 - 6%, polymethylene polyphenyl polyisocyanate and MDI polymer 6 - 25%, uretidione 1 - 5%; The polymethylene polyphenyl polyisocyanate is selected from at least one of isocyanates containing 6 or more benzene rings; Preferably, the number-average molecular weight of the MDI polymer is 450 - 10,000; Preferably, the bottom temperature of the rectification or distillation operation is 190 - 230 °C, more preferably 195 - 220 °C.

5. The preparation method according to claim 3 or 4, characterized in that the top pressure of the rectification or distillation operation is 20 - 1000 Pa (absolute pressure), preferably 300 - 800 Pa (absolute pressure); Preferably, the residence time of the crude MDI in the bottom of the rectification or distillation operation is 0.5 - 60 min.

6. The preparation method according to any one of claims 3 - 5, characterized in that when the mass percentage content of uretidione in the crude MDI exceeds 2%, the bottom temperature of the rectification or distillation operation is 200 - 220 °C; Preferably, when the mass percentage content of uretidione in the crude MDI exceeds 2%, the residence time of the crude MDI in the bottom of the rectification or distillation operation is 20 - 60 min; Preferably, the reflux ratio in the rectification operation is 0.05 - 1.

7. The preparation method according to any one of claims 3 - 6, characterized in that the temperature of the bottom product is cooled to below 90 °C within 120 s, preferably cooled to below 85 °C within 60 s, more preferably cooled to below 80 °C within 20 s through the cooling.

8. The preparation method according to any one of claims 3 - 7, characterized in that the cooling method includes at least one of direct mixing heat exchange or wall - type heat exchange; Preferably, the cold fluid for the direct mixing heat exchange is low - temperature polymerized MDI, and the composition of the low - temperature polymerized MDI is the same as that of the polymerized MDI.

9. The preparation method according to claim 8, characterized in that the temperature of the low - temperature polymerized MDI is 45 - 60 °C; Preferably, in the direct mixing heat exchange, the mass flow ratio of the low - temperature polymerized MDI to the bottom product is (3 - 10):1, more preferably (5 - 8):1; Preferably, the heat exchange equipment for the wall - type heat exchange includes any one of plate heat exchangers, spiral plate heat exchangers or shell - and - tube heat exchangers.

10. The preparation method according to any one of claims 3 - 9, characterized in that the preparation method specifically includes: part of methylene diphenyl diisocyanate is distilled out from the crude MDI through rectification or distillation operation according to a separation ratio of (0.05 - 1.5):1, and then the bottom product is cooled to obtain the polymerized MDI; the crude MDI includes first crude MDI or second crude MDI; The preparation method of the first crude MDI includes the following steps: (1) Aniline and formaldehyde react under the catalysis of hydrochloric acid to obtain crude MDA; The mass ratio of aniline, formaldehyde and hydrochloric acid is 1:(0.3 - 0.7):(0.05 - 0.5); The temperature of the reaction is 30 - 150 °C; The time of the reaction is 1 - 6 h; (2) Mix the crude MDA, inert solvent and phosgene and carry out a phosgenation reaction to obtain a phosgenation reaction solution; The temperature of the phosgenation reaction is 50 - 200 °C; The time of the phosgenation reaction is 1 - 60 min; The mass ratio of the crude MDA, inert solvent to phosgene is 1:(2 - 4):(2 - 6); (3) After the phosgenation reaction solution is subjected to phosgene removal and solvent removal, the first crude MDI is obtained; The second crude MDI is obtained after the first crude MDI is stored; The storage time is 0.1 - 6 months; The storage temperature is 10 - 50 °C; The separation ratio is the ratio of the mass of the distilled methylene diphenyl diisocyanate to the mass of the bottom product; The bottom temperature of the rectification or distillation operation is 190 - 230 °C, more preferably 195 - 220 °C; The top pressure of the rectification or distillation operation is 20 - 1000 Pa (absolute pressure), preferably 300 - 800 Pa (absolute pressure); The residence time of the crude MDI in the bottom of the rectification or distillation operation is 0.5 - 60 min; When the mass percentage content of uretidione in the crude MDI exceeds 2%, the bottom temperature of the rectification or distillation operation is 200 - 220 °C; When the mass percentage content of uretidione in the crude MDI exceeds 2%, the residence time of the crude MDI in the bottom of the rectification or distillation operation is 20 - 60 min; The reflux ratio in the rectification operation is 0.05 - 1; The temperature of the bottom product is cooled to below 90 °C within 120 s, preferably cooled to below 85 °C within 60 s, and more preferably cooled to below 80 °C within 20 s; The cooling method includes at least one of direct mixing heat exchange or wall heat exchange; The cold fluid for the direct mixing heat exchange is low-temperature polymeric MDI, and the composition of the low-temperature polymeric MDI is the same as that of the polymeric MDI; The temperature of the low-temperature polymeric MDI is 45 - 60 °C; The mass flow ratio of the low-temperature polymeric MDI to the bottom product is (3 - 10):1, preferably (5 - 8):1.

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