Polymeric MDI and preparation method therefor

Abstract

A polymeric MDI and a preparation method therefor. 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, 8-30% of polymethylene polyphenyl polyisocyanate and MDI polymer, and 1-2% of uretdione. The polymethylene polyphenyl polyisocyanate is selected from at least one of isocyanates containing six or more phenyl groups. When key indexes of the polymeric MDI such as isocyanate content, viscosity, and NCO mass content, are all normal, the content of uretdione is only 1-2%, the content of uretdione can be further controlled at 1.41-1.92%, and the shelf life at room temperature can be extended to one year or more.

Description

A kind of polymeric MDI and preparation method thereof Technical Field

[0001] The present application 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 is the abbreviation of polyphenylpolymethylene polyisocyanate, which is a widely used isocyanate and an important raw material for the preparation of polyurethane materials. It is widely used in refrigerators, freezers, artificial boards, building insulation, adhesives and other fields.

[0003] At present, polymeric MDI is mainly prepared by the phosgene method both at home and abroad. The process steps are briefly described as follows: aniline and formaldehyde undergo a condensation reaction under the catalytic action of an acid catalyst to produce a mixture of diphenylmethylenediamine and polyphenylmethylenepolyamines, called crude MDA. The crude MDA is mixed with an inert solvent and then phosgenated with phosgene to obtain a photochemical reaction liquid. After the phosgene and solvent are removed from the photochemical reaction liquid, a crude isocyanate product is obtained, called crude MDI. Crude MDI is a mixture of dicyclic and polycyclic isocyanates. The dicyclic MDI is evaporated out in a certain proportion through distillation or rectification of the crude MDI, and the product extracted from the bottom of the tower is polymeric MDI.

[0004] Polymeric MDI is liquid at room temperature and its normal storage temperature is 20-30°C. During storage, it slowly undergoes self-polymerization to form uretdione. This reaction is a reaction in which the NCO groups at each end of two MDI molecules combine to form a -NCNC- four-membered ring. The self-polymerization reaction is: Uretdione has a certain solubility in polymeric MDI. As the storage time increases, the uretdione content gradually increases. When the saturated precipitation point is reached, uretdione will precipitate in the form of a solid, causing the polymeric MDI to become turbid and unable to be used.

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

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

[0007] The uretdione content in polymeric MDI produced using existing technology is generally around 2.2-2.4%. However, when stored at room temperature, the uretdione content increases at a rate of 0.06% per month. Because its precipitation point is around 2.5%, its shelf life is generally less than six months. Therefore, reducing the initial uretdione content in polymeric MDI upon production can effectively extend its shelf life. Currently, no publicly available reports on control values ​​for the uretdione content in polymeric MDI, as well as specific control processes and methods, have been found in publicly available polymeric MDI production technologies, both domestically and internationally.

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

[0009] Summary of the Invention

[0010] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0011] In view of the shortcomings of the prior art, the purpose of this application is to provide a polymeric MDI with improved storage stability and a preparation method thereof, by reducing the initial uretdione content in the polymeric MDI to extend its shelf life, so that the storage time at room temperature can reach more than 6 months, or even more than one year.

[0012] To achieve this goal, this application adopts the following technical solutions:

[0013] In a first aspect, the present application provides a polymeric MDI, which comprises the following components, calculated by mass percentage: 20-65% methylene diphenyl diisocyanate, 15-40% methylene triphenyl triisocyanate, 5-20% trimethylene tetraphenyl tetraisocyanate, 1-8% tetramethylene pentaphenyl pentaisocyanate, 8-30% polymethylene polyphenyl polyisocyanate and MDI polymer, and 1-2% uretdione; the polymethylene polyphenyl polyisocyanate is selected from at least one isocyanate containing 6 or more phenyl groups.

[0014] 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 specific values ​​between the above points. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific points included in the range.

[0015] The mass percentage of dimethyl 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 specific values ​​between the above points. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific points included in the range.

[0016] The mass percentage 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 specific values ​​between the above points. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific points included in the range.

[0017] The mass percentage 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 specific values ​​between the above points. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific points included in the range.

[0018] The mass percentage 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 specific values ​​between the above points. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific points included in the range.

[0019] The mass percentage of uretdione 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 specific values ​​between the above points. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific points included in the range.

[0020] The polymeric MDI provided in this application, when its key indicators such as viscosity, NCO mass content and different MDI contents are normal, can extend its storage time by reducing the initial uretdione content of the polymeric MDI, and the shelf life can be extended from 6 months to more than 1 year.

[0021] The following are optional technical solutions for this application, but are not intended to limit the technical solutions provided in this application. Through the following preferred technical solutions, the objectives and beneficial effects of this application can be better achieved and realized.

[0022] As an optional 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 specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0023] In one embodiment, the mass content of NCO in the polymeric MDI is 29-32%, for example, 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 specific values ​​between the above points. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific points included in the range.

[0024] In one embodiment, the mass percentage of uretdione 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 specific values ​​between the above points. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific points included in the range.

[0025] In one embodiment, the number average molecular weight of the MDI polymer is 450-10,000, for example, 450, 500, 1,000, 2,000, 3,000, 4,000, 5,000, 6,000, 7,000, 8,000, 9,000 or 10,000, as well as specific values ​​between the above points. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific points included in the range.

[0026] The MDI polymer is generated by self-polymerization of at least two MDI molecules. Exemplarily, the MDI polymer includes the following compounds:

[0027] In a second aspect, the present application provides a method for preparing polymeric MDI as described in the first aspect, the preparation method comprising: distilling off a portion of methylene diphenyl diisocyanate from crude MDI according to a separation ratio of (0.05-1.5):1 by rectification or distillation, and then cooling the bottom extract 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 bottom extract.

[0028] The rectification or distillation operation described in the present application is carried out in chemical equipment. For example, the chemical equipment includes but is not limited to a falling film evaporator, a scraped evaporator, a molecular still, a distillation tower, a rectification tower, etc., and the rectification tower includes a dividing wall rectification tower.

[0029] The crude MDI described in this application is a crude isocyanate product, and the crude MDI includes a first crude MDI or a second crude MDI; the preparation method of the first crude MDI includes a phosgene process, and the phosgene process includes the following steps:

[0030] (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 hours;

[0031] (2) mixing the crude MDA, an inert solvent, and phosgene, and then performing a phosgenation reaction to obtain a photochemical reaction liquid; 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 of chlorobenzene, o-dichlorobenzene, toluene, or xylene, or a combination of at least two thereof; the mass ratio of the crude MDA, the inert solvent, and the phosgene is 1:(2-4):(2-6);

[0032] (3) After the photochemical reaction liquid is subjected to phosgene removal and solvent removal, the first crude MDI is obtained; the specific operation of the phosgene removal is: removing unreacted phosgene in the photochemical reaction liquid by a heat treatment method, the heat treatment temperature is 150-170°C, and can further be 160-165°C; the heat treatment time is 10-30min, and can further be 25-28min; the phosgene content of the photochemical reaction liquid after the heat treatment is reduced to 80-200ppm; the specific operation of the solvent removal is: removing the solvent in the photochemical reaction liquid after the heat treatment by a vacuum removal method, the vacuum removal pressure is 1.2-40kPaA, and can further be 1.5-2kPaA; the vacuum removal temperature is 180-210°C, and can further be 195-205°C; the vacuum removal time is 10-20min, and can further be 12-18min.

[0033] In one embodiment, the second crude MDI is obtained by storing the first crude MDI.

[0034] In one embodiment, 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 specific point values ​​between the above point values. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0035] In one embodiment, 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 specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0036] In one embodiment, the crude MDI comprises the following components by mass percentage: 40-75% of methylene diphenyl diisocyanate, 10-30% of methylene 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 uretdione; the polymethylene polyphenyl polyisocyanate is selected from at least one isocyanate containing 6 or more phenyl groups.

[0037] 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%, as well as specific values ​​between the above points. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific points included in the range.

[0038] The mass percentage of the dimethylene triphenyl triisocyanate is 10-30%, for example, it can be 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28% or 30%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0039] The mass percentage of the 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%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0040] The mass percentage of the 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%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0041] The mass percentage of the polymethylene polyphenyl polyisocyanate and the MDI polymer is 6-25%, for example, it can be 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22% or 25%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0042] The mass percentage of the uretdione 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 specific values ​​between the above points. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific points included in the range.

[0043] In one embodiment, the number average molecular weight of the MDI polymer is 450-10,000.

[0044] In one embodiment, the bottom temperature of the distillation or rectification 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 specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0045] In one embodiment, the bottom temperature of the distillation or rectification 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 specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0046] Since the reaction of two MDI molecules self-polymerizing to form uretdione is a reversible reaction, the generation rate of uretdione accelerates significantly with increasing temperature, but eventually reaches an equilibrium value. This equilibrium value gradually increases below 180°C and gradually decreases above 180°C. That is, when the temperature exceeds 180°C, the reaction will proceed in reverse, that is, uretdione decomposes to form MDI. Therefore, the higher the temperature, the greater the degree of reverse equilibrium, that is, the lower the uretdione content.

[0047] During the phosgenation and solvent removal stages, crude MDI already contains a significant amount of uretdione, typically between 1.5% and 2.5%, due to the high temperatures experienced. Upon entering the distillation or rectification unit, based on the uretdione decomposition equilibrium theory, higher bottom temperatures are associated with a lower uretdione content. However, at higher temperatures, MDI undergoes another self-polymerization reaction, generating carbodiimide and carbon dioxide. This reaction not only degrades the quality of the polymerized MDI but also poses safety risks such as solidification and overpressure explosion. Therefore, a higher bottom temperature is not necessarily better.

[0048] In one embodiment, the top pressure of the rectification or distillation operation is 20-1000Pa (absolute pressure), for example, it can be 20Pa, 50Pa, 100Pa, 150Pa, 200Pa, 300Pa, 400Pa, 500Pa, 600Pa, 700Pa, 800Pa, 900Pa or 1000Pa, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0049] In one embodiment, 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 specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0050] In one embodiment, the residence time of the crude MDI in the bottom of the tower during the rectification or distillation operation is 0.5-60 min, for example, 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 specific values ​​between the above points. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific points included in the range.

[0051] In one embodiment, when the mass percentage of uretdione in the crude MDI exceeds 2%, the bottom temperature of the rectification or distillation operation is 200-220°C, for example, 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 specific values ​​between the above points. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific points included in the range.

[0052] In one embodiment, when the mass percentage of uretdione in the crude MDI exceeds 2%, the residence time of the crude MDI in the bottom of the tower during the rectification or distillation operation is 20-60 min, for example, 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 specific point values ​​between the above point values. Due to space limitations and for the sake of brevity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0053] The residence time of the crude MDI in the bottom of the tower is not the shorter the better, and needs to be controlled within an appropriate range, because the storage time of the crude MDI in the storage tank is uncertain. When crude MDI with a long storage time is used, the uretdione content therein is high, and may even exceed 5%. At this time, in order to reduce the uretdione content in the polymerized MDI product, the bottom temperature needs to be adjusted as high as possible, and 200-220°C can be selected. In addition, the residence time of the bottom of the tower needs to be extended to allow uretdione sufficient time to decompose. The optional residence time is controlled at 20-60 minutes. Although further extending the residence time is beneficial to the decomposition of uretdione, it will be accompanied by a reaction to generate carbodiimide, resulting in an increase in product viscosity and a decrease in NCO content, which is not conducive to product application.

[0054] In one embodiment, the reflux ratio in the distillation 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 specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0055] In one embodiment, the temperature of the bottom produced is cooled to below 90°C (e.g., 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C or 90°C) within 120s (e.g., 10s, 20s, 30s, 40s, 50s, 60s, 70s, 80s, 90s, 100s, 110s or 120s) by the cooling, and optionally cooled to below 85°C (e.g., 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C or 90°C) within 60s (e.g., 10s, 20s, 30s, 40s, 50s or 60s), and further optionally cooled to below 85°C (e.g., 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C or 85°C) within 20s (e.g., 2s, 4s, 6s, 8s, 10s, 12s, 14s, 16s) within 30s. s, 18s or 20s) to below 80°C (e.g., 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C or 80°C).

[0056] The bottom extract needs to be cooled to obtain the polymeric MDI. The cooling process is also an important stage for the increase of uretdione content. In the temperature range of 120-170°C, uretdione increases rapidly and has a high equilibrium point. The shorter the residence time in this range, the smaller the increase of uretdione. Therefore, it is necessary to control the cooling time to reduce the uretdione content in the polymeric MDI.

[0057] In one embodiment, the cooling method includes at least one of direct mixed heat exchange or partition heat exchange.

[0058] In one embodiment, the cold fluid of the direct mixed 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.

[0059] In one embodiment, the temperature of the low-temperature polymerized MDI is 45-60°C, for example, 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 specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0060] In one embodiment, in the direct mixed heat exchange, the mass flow ratio of the low-temperature polymerized MDI to the bottom output 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.

[0061] In one embodiment, in the direct mixed heat exchange, the mass flow ratio of the low-temperature polymerized MDI to the bottom output 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.

[0062] In order to achieve the purpose of rapid cooling, by controlling the temperature of low-temperature polymerized MDI and the mass flow ratio of low-temperature polymerized MDI to the bottom output, it can be ensured that the bottom output is cooled from above 190°C to below 90°C within 1-5 seconds, which basically does not bring the risk of increased uretdione content.

[0063] In one embodiment, the heat exchange equipment for the partitioning heat exchange comprises any one of a plate heat exchanger, a spiral plate heat exchanger or a shell and tube heat exchanger.

[0064] In the present application, the uretdione content in polymeric MDI can be controlled within 1-2% by simply adjusting the process parameters of the rectification or distillation operation without adding any additional equipment or additives, and the uretdione content can be further controlled within 1.41-1.92%.

[0065] In one embodiment, the preparation method specifically comprises: distilling off a portion of methylene diphenyl diisocyanate from crude MDI by rectification or distillation according to a separation ratio of (0.05-1.5):1, and then cooling the bottom extract to obtain the polymeric MDI.

[0066] The crude MDI includes a first crude MDI or a second crude MDI.

[0067] The preparation method of the first crude MDI comprises the following steps:

[0068] (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 hours;

[0069] (2) mixing the crude MDA, an inert solvent, and phosgene, and performing a phosgenation reaction to obtain a photochemical reaction liquid; the phosgenation reaction temperature is 50-200° C.; the phosgenation reaction time is 1-60 min; and the mass ratio of the crude MDA, inert solvent, and phosgene is 1:(2-4):(2-6);

[0070] (3) After the phosgene and solvent are removed from the photochemical reaction liquid, the first crude MDI is obtained.

[0071] The second crude MDI is obtained by storing the first crude MDI.

[0072] The storage time is 0.1-6 months; the storage temperature is 10-50°C.

[0073] The separation ratio is the ratio of the mass of the distilled methylene diphenyl diisocyanate to the mass of the bottom extract.

[0074] The bottom temperature of the rectification or distillation operation is 190-230°C, and can be further 195-220°C.

[0075] The top pressure of the rectification or distillation operation is 20-1000 Pa (absolute pressure), and can further be 300-800 Pa (absolute pressure).

[0076] The residence time of the crude MDI in the tower bottom during the rectification or distillation operation is 0.5-60 min.

[0077] When the mass percentage of uretdione in the crude MDI exceeds 2%, the bottom temperature of the rectification or distillation operation is 200-220° C., and the residence time of the crude MDI in the bottom of the rectification or distillation operation is 20-60 minutes.

[0078] The reflux ratio in the distillation operation is 0.05-1.

[0079] The temperature of the bottom extraction material is cooled to below 90° C. within 120 seconds by the cooling, and can be cooled to below 85° C. within 60 seconds, and can be further cooled to below 80° C. within 20 seconds.

[0080] The cooling method includes at least one of direct mixed heat exchange and partition heat exchange.

[0081] The cold fluid of the direct mixed 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.

[0082] The temperature of the low-temperature polymerization MDI is 45-60°C.

[0083] The mass flow ratio of the low-temperature polymerized MDI to the bottom output is (3-10):1, optionally (5-8):1.

[0084] Compared with the prior art, this application has the following beneficial effects:

[0085] The polymeric MDI provided in this application has a uretdione content as low as 1-2% while maintaining normal key indicators such as various isocyanate content, viscosity, and NCO mass content. The uretdione content can be further controlled at 1.41-1.92%, and the shelf life can be extended to more than 1 year.

[0086] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0087] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0088] FIG1 is a flow chart of the preparation process of polymeric MDI provided in Example 1. DETAILED DESCRIPTION

[0089] The technical solution of the present application will be further described below with reference to the accompanying drawings and through specific implementation methods. Those skilled in the art should understand that the embodiments are only for the purpose of helping to understand the present application and should not be regarded as specific limitations of the present application.

[0090] The sources of some components in the following examples and comparative examples are as follows:

[0091] Preparation Example 1

[0092] A crude MDI and a preparation method thereof, the preparation method comprising the following steps:

[0093] (1) Aniline, formaldehyde, and hydrochloric acid were added to a reactor in a mass ratio of 1:0.45:0.3, the reaction temperature was 60°C, and the reaction time was controlled to 2.3 h. After condensation reaction, crude MDA was obtained;

[0094] (2) mixing the crude MDA, chlorobenzene, and phosgene in a mixer at a mass ratio of 1:2.5:3.2, adding the mixture to a reactor, and performing a phosgenation reaction at 90° C. for 40 minutes to obtain a photochemical reaction solution;

[0095] (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 to 161° C., and the residence time of the photochemical reaction liquid in the bottom of the tower is 26 minutes. After the heat treatment, the phosgene content in the photochemical reaction liquid is 150 ppm. The photochemical reaction liquid after phosgene removal is then subjected to solvent removal at 2 kPaA at a temperature of 195° C. for 18 minutes to obtain the crude MDI.

[0096] The crude MDI comprises the following components by mass percentage: 60.8% methylene diphenyl diisocyanate, 13.1% methylene triphenyl triisocyanate, 6.8% trimethylene tetraphenyl tetraisocyanate, 3.2% tetramethylene pentaphenyl pentaisocyanate, 13.9% polymethylene polyphenyl polyisocyanate and MDI polymer, and 2.2% uretdione.

[0097] Preparation Example 2

[0098] A crude MDI and a preparation method thereof, the preparation method comprising the following steps:

[0099] (1) Aniline, formaldehyde, and hydrochloric acid were added to a reactor in a mass ratio of 1:0.5:0.32, the reaction temperature was 70°C, and the reaction time was controlled to 2 h. After condensation reaction, crude MDA was obtained;

[0100] (2) mixing the crude MDA, chlorobenzene, and phosgene in a mass ratio of 1:2.9:3.7 in a mixer, adding the mixture to a reactor, and performing a phosgenation reaction at 100° C. for 35 minutes to obtain a photochemical reaction solution;

[0101] (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. The residence time of the photochemical reaction liquid in the bottom of the tower is 30 minutes. After the heat treatment, the phosgene content of the photochemical reaction liquid is 120 ppm. The photochemical reaction liquid after phosgene removal is then subjected to solvent removal at 1.5 kPaA at a temperature of 199° C. for 16 minutes to obtain a first crude MDI. The first crude MDI is stored at 45° C. for 3 months to obtain the crude MDI.

[0102] The crude MDI comprises the following components by mass percentage: 55.8% methylene diphenyl diisocyanate, 13.6% methylene triphenyl triisocyanate, 7.2% trimethylene tetraphenyl tetraisocyanate, 4.1% tetramethylene pentaphenyl pentaisocyanate, 15.4% polymethylene polyphenyl polyisocyanate and MDI polymer, and 3.9% uretdione.

[0103] Preparation Example 3

[0104] A crude MDI and a preparation method thereof, which differ from Preparation Example 1 in that in step (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 160° C., the residence time of the photochemical reaction liquid in the bottom of the tower is 15 minutes, and after the heat treatment, the phosgene content of the photochemical reaction liquid is 160 ppm; then, the photochemical reaction liquid after phosgene removal is subjected to solvent removal at 2 kPaA at a temperature of 205° C. for 12 minutes to obtain the crude MDI.

[0105] The crude MDI comprises the following components by mass percentage: 60.9% methylene diphenyl diisocyanate, 13.2% methylene triphenyl triisocyanate, 7% trimethylene tetraphenyl tetraisocyanate, 3.2% tetramethylene pentaphenyl pentaisocyanate, 13.9% polymethylene polyphenyl polyisocyanate and MDI polymer, and 1.8% uretdione.

[0106] Example 1

[0107] A polymeric MDI comprising the following components, calculated by weight: 45.19% methylene diphenyl diisocyanate, 17.76% methylene triphenyl triisocyanate, 9.3% trimethylene tetraphenyl tetraisocyanate, 4.44% tetramethylene pentaphenyl pentaisocyanate, 21.9% polymethylene polyphenyl polyisocyanate and MDI polymer, and 1.41% uretdione. The polymeric MDI has a viscosity of 199 cP at 25°C and an NCO content of 31.1%.

[0108] The preparation process flow of the polymeric MDI is shown in Figure 1. The preparation method comprises: distilling off a portion of the methylene diphenyl diisocyanate from the crude MDI provided in Preparation Example 1 in a distillation tower at a separation ratio of 0.39:1, wherein the tower top pressure is 300 Pa (absolute pressure), the tower bottom temperature is 212°C, the reflux ratio is 0.3, the residence time of the crude MDI in the tower bottom is 20 minutes, and the tower bottom output is first rapidly cooled to 73°C in 5 seconds using direct mixed heat exchange. The cold fluid is the polymeric MDI at 50°C, and the mass flow ratio of the cold fluid to the tower bottom output is 6:1. The temperature is then cooled to 50°C through a conventional shell-and-tube heat exchanger to obtain the polymeric MDI.

[0109] Example 2

[0110] A polymeric MDI comprising the following components, calculated by weight: 33.54% methylene diphenyl diisocyanate, 21.7% methylene triphenyl triisocyanate, 12.5% ​​trimethylene tetraphenyl tetraisocyanate, 6.9% tetramethylene pentaphenyl pentaisocyanate, 23.8% polymethylene polyphenyl polyisocyanate and MDI polymer, and 1.56% uretdione. The polymeric MDI has a viscosity of 590 cP at 25°C and an NCO content of 30.6%.

[0111] The preparation method of polymeric MDI includes: distilling off a portion of methylene diphenyl diisocyanate from the crude MDI provided in Preparation Example 2 in a distillation tower at a separation ratio of 0.54:1, wherein the tower top pressure is 350 Pa (absolute pressure), the tower bottom temperature is 216° C., the reflux ratio is 0.42, the residence time of the crude MDI in the tower bottom is 50 minutes, the tower bottom output is first rapidly cooled to 75° C. in 5 seconds using direct mixed heat exchange, the cold fluid is the polymeric MDI at 50° C., the mass flow ratio of the cold fluid to the tower bottom output is 5.6:1, and then the temperature is cooled to 50° C. through a conventional shell-and-tube heat exchanger to obtain the polymeric MDI.

[0112] Example 3

[0113] A polymeric MDI comprising the following components, calculated by weight: 51.25% methylene diphenyl diisocyanate, 16.7% methylene triphenyl triisocyanate, 8.7% trimethylene tetraphenyl tetraisocyanate, 4.1% tetramethylene pentaphenyl pentaisocyanate, 17.6% polymethylene polyphenyl polyisocyanate and MDI polymer, and 1.65% uretdione. The polymeric MDI has a viscosity of 140 cP at 25°C and an NCO content of 31.7%.

[0114] The preparation method of polymeric MDI includes: distilling off a portion of methylene diphenyl diisocyanate from the crude MDI provided in Preparation Example 1 in a distillation tower at a separation ratio of 0.25:1, wherein the tower top pressure is 300 Pa (absolute pressure), the tower bottom temperature is 208° C., the reflux ratio is 0.25, the residence time of the crude MDI in the tower bottom is 20 minutes, the tower bottom output is first rapidly cooled to 80° C. in 10 seconds using direct mixed heat exchange, the cold fluid is the polymeric MDI at 55° C., the mass flow ratio of the cold fluid to the tower bottom output is 5.1:1, and then the temperature is cooled to 55° C. through a conventional shell-and-tube heat exchanger to obtain the polymeric MDI.

[0115] Example 4

[0116] A polymeric MDI comprising the following components, calculated by weight: 27.54% methylene diphenyl diisocyanate, 23.5% methylene triphenyl triisocyanate, 12.4% trimethylene tetraphenyl tetraisocyanate, 7.2% tetramethylene pentaphenyl pentaisocyanate, 27.52% polymethylene polyphenyl polyisocyanate and MDI polymer, and 1.84% uretdione. The polymeric MDI has a viscosity of 750 cP at 25°C and an NCO content of 30.4%.

[0117] The preparation method of the polymeric MDI includes: distilling off a portion of the methylene diphenyl diisocyanate from the crude MDI provided in Preparation Example 2 in a distillation tower at a separation ratio of 0.67:1, wherein the tower top pressure is 350 Pa (absolute pressure), the tower bottom temperature is 210° C., the reflux ratio is 0.45, the residence time of the crude MDI in the tower bottom is 25 minutes, the tower bottom output is first rapidly cooled to 75° C. in 5 seconds using direct mixed heat exchange, the cold fluid is the polymeric MDI at 50° C., the mass flow ratio of the cold fluid to the tower bottom output is 5.4:1, and then the temperature is cooled to 50° C. through a conventional shell-and-tube heat exchanger to obtain the polymeric MDI.

[0118] Example 5

[0119] A polymeric MDI comprising the following components, calculated by weight: 51.01% methylene diphenyl diisocyanate, 16.66% methylene triphenyl triisocyanate, 8.7% trimethylene tetraphenyl tetraisocyanate, 4.1% tetramethylene pentaphenyl pentaisocyanate, 17.61% polymethylene polyphenyl polyisocyanate and MDI polymer, and 1.92% uretdione. The polymeric MDI has a viscosity of 143 cP at 25°C and an NCO content of 31.6%.

[0120] The preparation method of the polymeric MDI includes: distilling off a portion of the methylene diphenyl diisocyanate from the crude MDI provided in Preparation Example 3 in a distillation tower at a separation ratio of 0.25:1, wherein the tower top pressure is 300 Pa (absolute pressure), the tower bottom temperature is 192° C., the reflux ratio is 0.4, the residence time of the crude MDI in the tower bottom is 20 minutes, the tower bottom output is first rapidly cooled to 89° C. in 120 seconds using direct mixed heat exchange, the cold fluid is the polymeric MDI at 55° C., the mass flow ratio of the cold fluid to the tower bottom output is 3:1, and then the temperature is cooled to 55° C. through a conventional shell-and-tube heat exchanger to obtain the polymeric MDI.

[0121] Example 6

[0122] A polymeric MDI comprising the following components, calculated by weight: 20.1% methylene diphenyl diisocyanate, 26.5% methylene triphenyl triisocyanate, 14.1% trimethylene tetraphenyl tetraisocyanate, 7.99% tetramethylene pentaphenyl pentaisocyanate, 29.86% polymethylene polyphenyl polyisocyanate and MDI polymer, and 1.45% uretdione. The polymeric MDI has a viscosity of 990 cP at 25°C and an NCO content of 29% by weight.

[0123] The preparation method of polymeric MDI includes: distilling off a portion of methylene diphenyl diisocyanate from the crude MDI provided in Preparation Example 2 in a distillation tower at a separation ratio of 0.89:1, wherein the tower top pressure is 1000 Pa (absolute pressure), the tower bottom temperature is 220° C., the reflux ratio is 0.9, the residence time of the crude MDI in the tower bottom is 60 minutes, the tower bottom output is first rapidly cooled to 75° C. in 5 seconds using direct mixed heat exchange, the cold fluid is the polymeric MDI at 45° C., the mass flow ratio of the cold fluid to the tower bottom output is 5.2:1, and then the temperature is cooled to 50° C. through a conventional shell-and-tube heat exchanger to obtain the polymeric MDI.

[0124] Example 7

[0125] A polymeric MDI comprising the following components, calculated by weight: 43.5% methylene diphenyl diisocyanate, 16.9% methylene triphenyl triisocyanate, 9% trimethylene tetraphenyl tetraisocyanate, 4.25% tetramethylene pentaphenyl pentaisocyanate, 24.8% polymethylene polyphenyl polyisocyanate and MDI polymer, and 1.55% uretdione. The polymeric MDI has a viscosity of 220 cP at 25°C and an NCO content of 28.8%.

[0126] The only difference between the preparation method of polymeric MDI and Example 1 is that the residence time of the crude MDI in the bottom of the tower is 70 minutes. Other raw materials and process parameters are the same as those in Example 1.

[0127] Comparative Example 1

[0128] A polymeric MDI comprising the following components, calculated by weight: 45.48% methylene diphenyl diisocyanate, 17.3% methylene triphenyl triisocyanate, 9.12% trimethylene tetraphenyl tetraisocyanate, 4.32% tetramethylene pentaphenyl pentaisocyanate, 21.48% polymethylene polyphenyl polyisocyanate and MDI polymer, and 2.3% uretdione. The polymeric MDI has a viscosity of 200 cP at 25°C and an NCO content of 31% by weight.

[0129] The only difference between the preparation method of polymeric MDI and Example 1 is that the bottom product is first cooled by a common shell-and-tube heat exchanger and cooled to 90° C. in 3 minutes. Other raw materials and process parameters are the same as those in Example 1.

[0130] Comparative Example 2

[0131] A polymeric MDI comprising the following components, calculated by weight: 45.5% methylene diphenyl diisocyanate, 17.84% methylene triphenyl triisocyanate, 9.45% trimethylene tetraphenyl tetraisocyanate, 4.41% tetramethylene pentaphenyl pentaisocyanate, 20.7% polymethylene polyphenyl polyisocyanate and MDI polymer, and 2.1% uretdione. The polymeric MDI has a viscosity of 200 cP at 25°C and an NCO content of 31.1%.

[0132] The difference between the preparation method of polymeric MDI and Example 1 is that the bottom temperature is 186° C., the mass flow ratio of the cold fluid to the bottom output is 4.9:1, and other raw materials and process parameters are the same as those in Example 1.

[0133] Comparative Example 3

[0134] A polymeric MDI comprising the following components, calculated by weight: 33% methylene diphenyl diisocyanate, 21.7% methylene triphenyl triisocyanate, 12.5% ​​trimethylene tetraphenyl tetraisocyanate, 6.8% tetramethylene pentaphenyl pentaisocyanate, 23.5% polymethylene polyphenyl polyisocyanate and MDI polymer, and 2.5% uretdione. The polymeric MDI has a viscosity of 585 cP at 25°C and an NCO content of 29.3%.

[0135] The only difference between the preparation method of polymeric MDI and Example 2 is that the residence time of the crude MDI in the bottom of the tower is 15 minutes, the bottom temperature is 195° C., and the mass flow ratio of the cold fluid to the bottom output is 4.8:1. Other process parameters are the same as those in Example 2.

[0136] Product physical properties and performance testing:

[0137] (1) Determination of uretdione content: The determination was carried out by infrared spectroscopy. The specific steps were as follows: the sample to be tested was mixed with dichloromethane in a weight ratio of 1:30, dissolved evenly, and then transferred to an infrared liquid pool for infrared spectrum scanning. On the spectrum, the 1780 cm -1 and 1900cm -1 The obtained ratio is the content of uretdione in the sample to be tested;

[0138] (2) Determination of the content of each isocyanate and MDI polymer in crude MDI or polymerized MDI: High performance liquid chromatography (HPLC), model: Agilent 1200s, chromatographic column: Waters SymmetryShield RP18 5um, analysis method: 0.05g of the sample to be tested was dissolved in 2g of dichloromethane, and 2g of methanol was added for derivatization and analysis. The content of each isocyanate and MDI polymer in the sample to be tested was obtained by area normalization;

[0139] (3) Determination of viscosity of polymeric MDI: in accordance with GB / T 12009.3-2016 “Determination of viscosity of polymethylene polyphenyl polyisocyanate”;

[0140] (4) Determination of NCO content of polymeric MDI: in accordance with GB / T 12009.4-2016 “Determination of isocyanate content”.

[0141] The polymeric MDI provided in Examples 1-7 and Comparative Examples 1-3 were stored at 20° C. The uretdione content at different storage times was tested using the above test method, and the appearance was observed. The test results are shown in Table 1:

[0142] Table 1

[0143] As can be seen from Examples 1-6, the polymeric MDI provided by the present application is still clear and transparent in appearance after being stored at room temperature for one year, and its shelf life is extended. Compared with Example 1, the residence time of crude MDI in the tower bottom is extended in Example 7, and the initial uretdione content of the produced polymeric MDI does not change significantly, but the polymer content increases and the NCO content decreases, indicating that a too long residence time will cause the degree of polymerization of MDI in the tower bottom to deepen, thereby deteriorating the performance indicators of the product. As can be seen from the comparison of Comparative Example 1 with Example 1, the cooling time of the tower bottom extract is extended, which makes the initial uretdione content in the polymeric MDI higher, resulting in a shorter shelf life. As can be seen from the comparison of Comparative Example 2 with Example 1, when the uretdione content in the crude MDI is higher than 2%, the tower bottom temperature is too low, and the amount of uretdione decomposition is small, which makes the initial uretdione content in the polymeric MDI higher, resulting in a shorter shelf life. From the comparison between Comparative Example 3 and Example 2, it can be seen that when the uretdione content in the crude MDI is higher than 2%, the bottom temperature is too low and the bottom residence time is too short, which will cause the initial uretdione content in the polymerized MDI to be too high, resulting in a shorter shelf life.

[0144] The applicant declares that while the above-mentioned embodiments are used to illustrate the detailed process flow of the present application, the present application is not limited to the above-mentioned detailed process flow, which does not mean that the present application must rely on the above-mentioned detailed process flow in order to be implemented. Those skilled in the art should understand that any improvements to the present application, equivalent replacements for the raw materials of the present application's products, addition of auxiliary ingredients, and selection of specific methods, etc., fall within the scope of protection and disclosure of the present application.

Claims

1. A polymeric MDI, comprising 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%; Among them, The polymethylene polyphenyl polyisocyanate is selected from at least one of the isocyanates containing more than 6 benzene rings.

2. The polymeric MDI according to claim 1, wherein, The viscosity of the polymeric MDI at 25 °C is 50 - 1000 cP; Optionally, the mass content of NCO in the polymeric MDI is 29 - 32%; Optionally, the mass percentage content of uretidione in the polymeric MDI is 1 - 1.6%; Optionally, 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, comprising: The crude MDI is subjected to rectification or distillation operations to distill out part of 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, wherein, The crude MDI includes the first crude MDI or the second crude MDI; Optionally, 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) The crude MDA, inert solvent and phosgene are mixed and then subjected to 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; Optionally, the second crude MDI is obtained after the first crude MDI is stored; Optionally, the storage time is 0.1 - 6 months; Optionally, the storage temperature is 10 - 50 °C; Optionally, the crude MDI includes the following components by 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, 1 - 5% of uretidione; The polymethylene polyphenyl polyisocyanate is selected from at least one of the isocyanates containing more than 6 benzene rings; Optionally, the number average molecular weight of the MDI polymer is 450 - 10000; Optionally, the bottoms temperature of the rectification or distillation operation is 190 - 230 °C, and further optionally 195 - 220 °C.

5. The preparation method according to claim 3 or 4, wherein, The top pressure of the rectification or distillation operation is 20 - 1000 Pa (absolute pressure), and further optionally 300 - 800 Pa (absolute pressure); Optionally, the residence time of the crude MDI in the bottoms of the rectification or distillation operation is 0.5 - 60 min.

6. The preparation method according to any one of claims 3 - 5, wherein, 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; Optionally, 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; Optionally, the reflux ratio in the rectification operation is 0.05 - 1.

7. According to the preparation method described in any one of claims 3 to 6, wherein, The temperature of the column kettle product is cooled to below 90 °C within 120 s, optionally cooled to below 85 °C within 60 s, and further optionally cooled to below 80 °C within 20 s.

8. According to the preparation method described in any one of claims 3 to 7, wherein, The cooling method includes at least one of direct mixing heat exchange or wall - type heat exchange; Optionally, 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. According to the preparation method described in claim 8, wherein, The temperature of the low - temperature polymerized MDI is 45 - 60 °C; Optionally, in the direct mixing heat exchange, the mass flow ratio of the low - temperature polymerized MDI to the column kettle product is (3 - 10):1, and further optionally (5 - 8):1; Optionally, 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. According to the preparation method described in any one of claims 3 to 9, wherein, The preparation method specifically includes: the crude MDI is subjected to rectification or distillation operations to distill out part of methylene diphenyl diisocyanate according to a separation ratio of (0.05 - 1.5):1, and then the column kettle product is cooled to obtain the polymerized MDI; The crude MDI includes the first crude MDI or the 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) The crude MDA, inert solvent and phosgene are mixed and then subjected to 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 column kettle product; The column kettle temperature of the rectification or distillation operation is 190 - 230 °C, and further optionally 195 - 220 °C; The top pressure of the rectification or distillation operation is 20 - 1000 Pa (absolute pressure), and optionally 300 - 800 Pa (absolute pressure); The residence time of the crude MDI in the column kettle during the rectification or distillation operation is 0.5 - 60 min; 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; When the mass percentage content of uredione 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; The reflux ratio in the rectification operation is 0.05 - 1; The temperature of the column kettle product is cooled to below 90 °C within 120 s, optionally cooled to below 85 °C within 60 s, and further optionally 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 polymerized MDI, and the composition of the low-temperature polymerized MDI is the same as that of the polymerized MDI; The temperature of the low-temperature polymerized MDI is 45 - 60 °C; The mass flow ratio of the low-temperature polymerized MDI to the column kettle product is (3 - 10):1, and further optionally (5 - 8):1.

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