Dam preparation method beneficial to reducing content of diphenylmethane diamine

Abstract

The present invention provides a DAM preparation method beneficial to reducing the content of diphenylmethane diamine. During the preparation of DAM by using the method of the present invention, the condensation reaction characteristic index R is controlled to meet specific requirements, which makes it beneficial to obtaining a DAM product with an appropriate content of diphenylmethane diamine in DAM. The method comprises the following steps: (S1) bringing aniline into contact with at least a part of hydrochloric acid for a reaction to obtain aniline hydrochloride; (S2) subjecting the aniline hydrochloride to a condensation reaction with an aqueous formaldehyde solution to obtain a condensation reaction solution, wherein the aqueous formaldehyde solution is introduced by a single addition or by multiple additions, and in step (S2), a part of hydrochloric acid is optionally added; and (S3) subjecting the condensation reaction solution to a transposition reaction to obtain a transposition reaction solution, wherein in step (S3), a part of hydrochloric acid is optionally added; and during steps (S1) to (S3), the condensation reaction characteristic index R is controlled to satisfy 1<R<1.05, more preferably 1.01<R<1.04.

Description

Method for preparing DAM that is beneficial to reducing diphenylmethanediamine content Technical Field

[0001] The present invention relates to the technical field of preparation of diphenylmethane series diamines and polyamines (DAM), and in particular to a method for preparing DAM by reducing the content of diphenylmethane diamine in DAM. Background Art

[0002] Diphenylmethane series diamines and polyamines (abbreviated as DAM) refer to mixtures of the following types of diamines and polyamines:

[0003] Where n represents a natural number ≥ 0. DAM can be phosgenated to convert all -NH2 groups in the above structure into -NCO groups, thereby generating polyphenylmethane polyisocyanate (PMDI). This PMDI can then be fractionated to separate a portion of diphenylmethane diisocyanate (MDI), yielding a PMDI product with the desired viscosity.

[0004] Because the market demand for MDI products is significantly affected by season, demand for MDI in some countries and regions is low or even nonexistent during the off-season. Furthermore, due to the relatively short shelf life of MDI products, long-term storage not only affects the quality of the MDI products but also leads to inventory backlogs. Therefore, it is necessary to develop a method to reduce the proportion of MDI separation in PMDI. This would minimize the proportion of MDI products in PMDI during the off-season, or even eliminate the need for MDI separation. This would ensure that the PMDI obtained from the phosgenation of DAM has an appropriate viscosity and can be used directly as a product.

[0005] In the traditional MDI production process, formaldehyde and aniline undergo condensation and metathesis reactions in the presence of hydrochloric acid, followed by neutralization, water washing, and distillation to produce DAM. DAM then undergoes phosgenation to produce PMDI. Since the ratio of the various rings in PMDI cannot be adjusted during the phosgenation process, adjusting the yield of MDI products requires adjusting the ratio of the various rings in PMDI. Therefore, process parameters can only be adjusted during the condensation reaction stage to achieve DAM with a specific ring ratio.

[0006] During the condensation reaction, methods for regulating the ratio of the various ring contents in DAM primarily include adjusting the molar ratio of formaldehyde to aniline or the molar ratio of hydrochloric acid to aniline, which is a well-known method in the industry. However, if these methods are used alone to achieve DAM production sufficient for direct PMDI production, the molar ratio of formaldehyde to aniline would need to be significantly increased or the molar ratio of hydrochloric acid to aniline would need to be significantly reduced. However, increasing the molar ratio of formaldehyde to aniline exacerbates condensation side reactions, leading to an increase in the N-methyl impurity content in DAM. Furthermore, the viscosity of the condensation reaction liquid increases, exacerbating blockages in the reactor and heat exchanger during operation, impacting the long-term stable operation of the device. Furthermore, hydrochloric acid acts as a catalyst for the transposition and rearrangement of the intermediate product (aminobenzylamine) in the condensation reaction. As the molar ratio of hydrochloric acid to aniline decreases, this affects the rate of transposition and rearrangement of aminobenzylamine to DAM, leading to an increase in the aminobenzylamine content in DAM. This substance, upon phosgenation, produces PI (phenyl isocyanate), which can cause blockage in the vacuum unit and affect the stable operation of the system.

[0007] Patent US4792624A describes a method in which a portion of DAM is mixed with the reaction liquid from the condensation stage and reintroduced into the condensation reaction. The mass ratio of this DAM to aniline is 1-40%. By varying the DAM circulation rate, the DAM composition, and thus the PMDI product composition, can be regulated. However, during the continuous reaction process, the continuous circulation of DAM within the condensation reaction system causes the accumulation of impurities such as N-methyl compounds within the DAM, resulting in a DAM product with lower quality than that obtained using conventional processes. Furthermore, this process also significantly increases the content of polycyclic MDA in the DAM, exacerbating the formation of macromolecular blockages during production and affecting the long-term stable operation of the system.

[0008] Unlike the aforementioned patents, patent WO2023063852A1 proposes a new method that separates and partially separates dicyclic MDA from DAM by distillation. This dicyclic MDA is then recycled and used in a condensation reaction to obtain DAM with a high tricyclic MDA content. This allows the final PMDI to meet the required viscosity without the need for additional MDI separation. However, separating dicyclic MDA from DAM at the recycling ratio mentioned in the patent (0-25%) would result in significant energy consumption, making this method less economical.

[0009] Summary of the Invention

[0010] The present invention provides a method for preparing DAM (diamines and polyamines of the diphenylmethane series) that is conducive to reducing the content of diphenylmethane diamine in DAM. By using the method of the present invention to prepare DAM, by controlling the dosage ratios of the main raw materials, the condensation reaction, and the addition ratios of hydrochloric acid / formaldehyde in each reaction step and other factors to meet the specific requirements of the condensation reaction characteristic index R, it is conducive to obtaining a DAM product with an appropriate content of diphenylmethane diamine in DAM.

[0011] To achieve its purpose, the present invention provides the following technical solutions:

[0012] The present invention provides a method for preparing DAM (diamines and polyamines of the diphenylmethane series) that is conducive to reducing the content of diphenylmethane diamine, and the steps of the method include:

[0013] (S1) Aniline reacts with at least part of hydrochloric acid to obtain aniline hydrochloride;

[0014] (S2) The aniline hydrochloride is subjected to a condensation reaction with an aqueous formaldehyde solution to obtain a condensation reaction solution, and the aqueous formaldehyde solution is added once or in multiple portions; optionally, part of hydrochloric acid is added in step (S2);

[0015] (S3) The condensation reaction solution is subjected to a rearrangement reaction to obtain a rearrangement reaction solution; optionally, part of hydrochloric acid is added in step (S3);

[0016] During the process of performing steps (S1) to (S3), control the condensation reaction characteristic index R to satisfy 1 < R < 1.05, more preferably 1.01 < R < 1.04, where the R is calculated by the following formula (I):

[0017] R = 0.023(n1 + (-1.158N + - 0.025N + 0.528)) × (0.0023T 2 - 0.141T + 45.3) (I)

[0018] In formula (I), the N (which can be called the hydrochloric acid ratio index) is calculated by the following formula (II):

[0019] N = n2 × (a1 + (0.32b + 0.64)a2 + 0.64a3) (II)

[0020] In formula (I), n1 is the molar ratio of the total amount of formaldehyde used to the total amount of aniline used, and T is the reaction temperature of step (S2); in formula (II), n2 is the molar ratio of the total amount of hydrochloric acid calculated as H + to the total amount of aniline used, a1 is the molar proportion of the amount of hydrochloric acid calculated as H + used in step (S1) to the total amount of hydrochloric acid used, a2 is the amount of hydrochloric acid calculated as H+ The molar ratio of the hydrochloric acid dosage calculated based on H in step (S3) to the total hydrochloric acid dosage, and a3 is the molar ratio of the hydrochloric acid dosage calculated based on H in step (S3) to the total hydrochloric acid dosage, and b is the molar ratio of the formaldehyde added for the first time in step (S2) to the total formaldehyde amount. + In the present invention, the condensation reaction characteristic index R is controlled to satisfy 1 < R < 1.05, for example, R is 1.002, 1.005, 1.007, 1.010, 1.015, 1.020, 1.025, 1.030, 1.035, 1.038, 1.039, 1.040 or 1.045, etc., and more preferably 1.01 < R < 1.04.

[0021]

[0022] Through a large number of studies, the inventor of the present invention found that by controlling 1 < R < 1.05, more preferably 1.01 < R < 1.04, during the preparation of DAM, a DAM product with neither too high nor too low diphenylmethane diamine content can be obtained, which is beneficial for directly obtaining PMDI with an appropriate viscosity through a phosgenation reaction without an additional MDI separation process. For example, DAM with a diphenylmethane diamine content of 45wt% - 50wt% can be obtained, and PMDI with a viscosity of 150 - 250 mPa·S (25°C) can be obtained without adding an MDI separation process. By using the preparation method of the present invention to prepare DAM, through the coordinated adjustment of various parameters, excessive adjustment of a single parameter can be avoided, which is beneficial for the device system to operate under relatively mild parameter conditions, beneficial for increasing the long-term stable operation period of the system, and also avoiding the subsequent rectification separation link. The separation of bicyclic MDI can be carried out without a rectification process, which can reduce equipment costs and energy consumption and improve economic benefits. By preparing DAM in a preferred manner and preparing PMDI through a phosgenation reaction based on this DAM, the obtained PMDI can directly obtain a PMDI product with an appropriate viscosity without further rectifying and separating the additional MDI. The inventor of the present invention found that if the R value is too high, the content of bicyclic MDI in the DAM product will be too low, and finally a PMDI product with too high viscosity will be obtained, and the impurity content in the DAM obtained when the R value is too high will also be significantly too high; if the R value is too low, the content of bicyclic MDI in the DAM product will be too high, and a PMDI product with too low viscosity will be obtained without an MDI separation process.

[0023] ​In some embodiments, in step (S2), the aniline hydrochloride is mixed with a portion of the formaldehyde aqueous solution to perform a first-stage condensation reaction; then, a portion of the formaldehyde aqueous solution and, optionally, a portion of the hydrochloric acid are added to continue the second-stage condensation reaction to obtain the condensation reaction solution; the temperature of the first-stage condensation reaction and the temperature of the second-stage condensation reaction are the same or different. When the temperature is different, the value of T in formula (I) is the logarithmic mean of the condensation reaction temperatures of each stage, i.e., the logarithmic mean of the reaction temperatures of the first and second stages; in step (S3), the condensation reaction solution is mixed with a portion of the hydrochloric acid, and then the transposition reaction is performed. This preferred method for preparing DAM facilitates better results.

[0024] Furthermore, the preparation method further comprises the following steps:

[0025] (S4) neutralizing the transposition reaction liquid, and then separating the phases to obtain an organic phase, and the organic phase is washed with water and then refined to obtain the DAM. The phase separation and water washing operations can be carried out using processes known in the art, and there are no special restrictions on this. Specifically, neutralization is, for example, neutralizing the transposition reaction liquid with an alkali solution at 60-120°C, and the alkali solution is, for example, a sodium hydroxide aqueous solution with a concentration of 10-50wt%, and the molar ratio of sodium hydroxide to hydrochloric acid (calculated as HCl) is, for example, 1.01-1.20. Refining is, for example, performing a vacuum distillation treatment on the washed organic phase at a pressure of 0.1-10kPa and a temperature of 150-250°C to remove aniline and water therein.

[0026] In some embodiments, the reaction temperature T in step (S2) is 40-100°C, for example, 40°C, 45°C, 60°C, 70°C, 80°C, 90°C, 95°C or 100°C, etc., preferably 45-95°C, and the total reaction time of step S2 is 30-120min, for example, 40min, 50min, 60min, 70min, 80min, 90min, 100min, 110min.

[0027] In some embodiments, the molar ratio n1 of the total amount of formaldehyde to the total amount of aniline is 0.2-0.6, for example, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55 or 0.60, etc., preferably 0.3-0.55.

[0028] In some embodiments, the H + The molar ratio n2 of the total amount of hydrochloric acid used and the total amount of aniline used is 0.01-0.4, for example 0.01, 0.03, 0.05, 0.10, 0.20, 0.30 or 0.40, etc., preferably 0.05-0.3.

[0029] In some embodiments, a1, a2, and a3 satisfy the following requirements: 0 < a1 ≤ 1, 0 ≤ a2 < 1, 0 ≤ a3 < 1, and a1 + a2 + a3 = 1. For example, a1 is 0.05, 0.07, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, or 1.00. For example, a2 is 0, 0.05, 0.07, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, or 0.90. For example, a3 is 0, 0.05, 0.07, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, or 0.90. Preferably, a1 is 0.1-1, a2 is 0-0.4, and a3 is 0-0.8. Adopting the preferred ratio is conducive to obtaining a DAM product with a relatively low impurity content.

[0030] In some embodiments, the molar ratio b of the formaldehyde added for the first time in step (S2) to the total formaldehyde is 0.01-1, for example, 0.01, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, 0.95 or 1.00, etc.; preferably 0.01-0.99, more preferably 0.2-0.8;

[0031] In the present invention, the formaldehyde solution can be added to the step (S2) in a single addition manner, that is, all the formaldehyde solution is added to the reaction system by a single addition operation; the formaldehyde solution can also be added to the step (S2) in a multiple addition manner, so that the condensation reaction is carried out in multiple stages, that is, each addition of the formaldehyde solution corresponds to a condensation reaction stage, and when the formaldehyde solution is added multiple times, each formaldehyde solution is reacted for a period of time after being added, and then the next formaldehyde solution is added. The specific operation of adding the formaldehyde solution each time is not particularly limited, and can be a common formaldehyde solution addition operation in the art, such as adding the formaldehyde solution by mixing in a stirred tank, a static mixer, etc. Preferably, the formaldehyde solution in step (S2) is added 2-5 times, for example, the formaldehyde solution is added 2, 3, 4 or 5 times.

[0032] In some embodiments, the mass concentration of the hydrochloric acid used is 25-37wt%, for example, 26wt%, 28wt%, 32wt%, 34wt%, etc., preferably 30-35wt%, and the mass concentration of the formaldehyde aqueous solution used is 20-55wt%, for example, 25wt%, 30wt%, 35wt%, 37wt%, 40wt%, 44wt%, 50wt%, 52wt%, etc., preferably 30-50wt%.

[0033] In some embodiments, in the method for preparing DAM, the reaction temperature T in step (S2) is controlled to be 40 - 100 °C, preferably 45 - 95 °C; the molar ratio n1 of the total amount of formaldehyde used to the total amount of aniline used is controlled to be 0.2 - 0.6, preferably 0.3 - 0.55; the molar ratio n2 of the total amount of hydrochloric acid used calculated based on H + to the total amount of aniline used is controlled to be 0.01 - 0.4, preferably 0.05 - 0.3; the molar proportion b of the formaldehyde added for the first time in step (S2) to the total amount of formaldehyde is controlled to be 0.01 - 1, preferably 0.01 - 0.99, more preferably 0.2 - 0.8; a1, a2, and a3 are controlled to meet the following requirements respectively: 0 < a1 ≤ 1, 0 ≤ a2 < 1, 0 ≤ a3 < 1, and a1 + a2 + a3 = 1. Preferably, a1 is 0.1 - 1, a2 is 0 - 0.4, and a3 is 0 - 0.8; on this basis, the condensation reaction characteristic index R is simultaneously controlled to satisfy 1 < R < 1.05, more preferably 1.01 < R < 1.04. The DAM prepared by this embodiment has an appropriate content of diphenylmethane diamine (i.e., dicyclo MDI) (the content is, for example, between 45 - 50%), and can take into account the relatively low content of N-methyl impurities and / or MDA with more than ten rings and / or impurities such as aminobenzylamine. Based on this DAM to prepare PMDI, a PMDI product with appropriate viscosity (the viscosity at 25 °C is, for example, 150 - 250 mPa·S) can be obtained without additional operation for removing dicyclo MDI.

[0034] In some embodiments, in step (3), the reaction temperature for the rearrangement reaction is 100 - 150 °C, such as 105 °C, 110 °C, 115 °C, 120 °C, 125 °C, 130 °C, 135 °C, 140 °C, 145 °C, etc.; the reaction time is 30 - 300 min, such as 60 min, 120 min, 150 min, 180 min, 240 min, 270 min, etc.

[0035] By the method of the present invention, a DAM product with an appropriate content of dicyclo MDA can be obtained. For example, the mass content of diphenylmethane diamine in the obtained DAM is 45% - 50%, such as 45%, 45.5%, 46%, 46.5%, 4,7%, 47.5%, 48%, 48.5%, 49%, 49.7%, etc.

[0036] In the text, diphenylmethane diamine refers to 4,4'-MDA, 2,4'-MDA, and / or 2,2'-MDA. Among them, the "content of diphenylmethane diamine" mentioned in the text refers to the mass proportion of the total amount of 4,4'-MDA, 2,4'-MDA, and 2,2'-MDA in DAM.

[0037] In this article, "N-methyl impurities" refer to polyamines (i.e., diamines and polyamines of the diphenylmethane series) with a methyl group replacing a hydrogen atom on the amino group. These polyamines can have two, three, or multiple benzene rings, with N-methyl impurities primarily occurring in polyamines with two benzene rings. "Deca-ring or higher MDA" impurities refer to polyphenylmethane polyamines with ten or more benzene rings (hereinafter referred to as deca-ring or higher polyamines).

[0038] A second aspect of the present invention provides a method for preparing PMDI (polyphenylmethane polyisocyanate), wherein DAM undergoes a phosgenation reaction with phosgene, and the PMDI is obtained after solvent removal. The DAM is produced by the method described above, and after the phosgenation reaction, no MDI removal operation is required, i.e., no excess MDI removal step is involved. Preferably, the PMDI has a viscosity of 150-250 mPa·s at 25°C, for example, 150 mPa·s, 170 mPa·s, 200 mPa·s, 220 mPa·s, 230 mPa·s, or 250 mPa·s. The process for preparing PMDI by the phosgenation reaction of DAM with phosgene can be carried out using corresponding processes known in the art, such as those described in patent applications CN102070491B, CN108147979B, DE1192641A, etc., and will not be described in detail here. The key point of the present invention is that PMDI is prepared by using DAM prepared by the method of the present invention, without the need to add an MDI removal operation as in the traditional process.

[0039] The technical solution provided by the present invention has the following beneficial effects:

[0040] The method of the present invention can achieve the preparation of DAM with an appropriate diphenylmethanediamine content by controlling the condensation reaction characteristic index R to meet specific requirements. This can avoid the disadvantages of traditional processes that achieve this goal by significantly adjusting a single parameter. For example, it is not necessary to obtain DAM with a relatively appropriate diphenylmethanediamine content under the conditions of an excessively high formaldehyde ratio or an excessively low hydrochloric acid ratio. This can improve the quality of the condensation reaction and simultaneously reduce the contents of N-methyl impurities, aminobenzylamine and / or macromolecular polymer impurities in the relatively low DAM, thereby improving the operational stability of the system and alleviating the clogging problem of the device during long-term operation.

[0041] The method of the present invention is used to prepare DAM, which is beneficial to improving the reaction quality of the device during actual production operation, and can also simplify the separation process of PMDI prepared downstream, reducing the equipment and energy consumption costs of MDI separation. DETAILED DESCRIPTION

[0042] For the convenience of understanding the present invention, the present invention will be further described below in conjunction with embodiments. It should be understood that the following embodiments are only for better understanding the present invention, and do not mean that the present invention is only limited to the following embodiments.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The term "and / or" as used herein may include any and all combinations of one or more of the related listed items.

[0044] For those parts in the embodiments where specific experimental steps or conditions are not specified, the operations or conditions of the corresponding conventional experimental steps in the technical field can be followed. For reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchases.

[0045] <Raw material source>

[0046] Aniline: provided by Ningbo Wanhua Industrial Park, with a concentration of 99.9 wt%;

[0047] Hydrochloric acid: provided by Ningbo Wanhua Industrial Park, with a concentration of 33 wt%;

[0048] Formaldehyde aqueous solution: provided by Ningbo Wanhua Industrial Park, with a concentration of 37 wt%;

[0049] Sodium hydroxide aqueous solution: provided by Ningbo Wanhua Industrial Park, with a concentration of 50 wt%;

[0050] <Testing method>

[0051] The determination method of each component in DAM is carried out by liquid chromatography, and the analytical instrument is Agilent 1200;

[0052] The determination method of PMDI viscosity: measured by a digital viscometer, and the analytical instrument is a Nyrun DV-79 model viscometer.

[0053] In the following embodiments, the content of dicyclo MDA, that is, the content of diphenylmethane diamine, refers to the mass ratio of the total amount of 4,4'-MDA, 2,4'-MDA and 2,2'-MDA in DAM.

[0054] In the following embodiments, during the preparation of DAM, the condensation reaction characteristic index R is controlled to satisfy 1 < R < 1.05, and more preferably 1.01 < R < 1.04. For the calculation formula of the condensation reaction characteristic index R, refer to the previous formulas (I) and (II), which will not be elaborated here.

[0055] In the following text, unless otherwise specified, the molar ratio of hydrochloric acid to aniline refers to the molar ratio of hydrochloric acid to aniline calculated based on H + 1]]calculated.

[0056] In the following examples and comparative examples, PMDI was prepared from DAM by phosgenation reaction mainly according to the process flow of Example 1 in patent CN108147979B.

[0057] Example 1

[0058] (S1) Mix the first step hydrochloric acid (33% wt) and aniline (99.99% wt), hydrochloric acid (H + The molar ratio of aniline to aniline was 0.048 to obtain aniline hydrochloride reaction solution.

[0059] (S2) Then, a first portion of the formaldehyde aqueous solution (37% wt) is gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction. The molar ratio of formaldehyde to aniline in the first portion of the formaldehyde aqueous solution is 0.20. The addition time of this formaldehyde aqueous solution is controlled to be 20 minutes, and the reaction temperature is maintained at 60°C during this period.

[0060] When the cumulative molar ratio of formaldehyde to aniline reached 0.20, a second portion of hydrochloric acid was added to the reaction solution. The addition time for this hydrochloric acid addition was controlled to 5 minutes, with a molar ratio of hydrochloric acid to aniline of 0.024. The reaction temperature was maintained at 60°C. After the addition of the second portion of hydrochloric acid was complete, a second portion of formaldehyde aqueous solution was added to the reaction system for condensation reaction. The molar ratio of formaldehyde to aniline in this formaldehyde aqueous solution was 0.29. The addition time was controlled to 25 minutes, with the reaction temperature maintained at 60°C. After the addition was complete, the reaction was maintained at 60°C for 30 minutes.

[0061] (S3) Then, a third portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.168. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 120° C. and the reaction is maintained for 120 minutes to carry out the transposition reaction.

[0062] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0063] In this example, the condensation reaction characteristic index R = 1.016. The DAM obtained according to the above production process had a dicyclic MDA content of 49.1%, an N-methyl impurity content of 0.15%, an aminobenzylamine content of 152 ppm, and a polyamine content of 10 or more rings of 78 ppm. The resulting DAM was subjected to phosgenation to produce PMDI. After solvent removal and without MDI removal, the PMDI viscosity was measured to be 173 mPa·s (25°C).

[0064] Example 2

[0065] (S1) Mix the first step hydrochloric acid (33% wt) and aniline (99.99% wt), hydrochloric acid (H + The molar ratio of aniline to aniline was 0.096 to obtain aniline hydrochloride reaction solution.

[0066] (S2) Then, a first portion of the formaldehyde aqueous solution (37% wt) was gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction. The molar ratio of formaldehyde to aniline in the first portion of the formaldehyde aqueous solution was 0.15. The addition time of this formaldehyde aqueous solution was controlled to be 20 minutes, and the reaction temperature was maintained at 70°C during the addition.

[0067] When the molar ratio of formaldehyde to aniline reached 0.15, a second portion of hydrochloric acid was added to the reaction solution. The addition time for this hydrochloric acid addition was controlled to 5 minutes, with a molar ratio of hydrochloric acid to aniline of 0.084. The reaction temperature was maintained at 70°C. After the addition of the second portion of hydrochloric acid was complete, a second portion of formaldehyde aqueous solution was added to the reaction system for condensation reaction. The molar ratio of formaldehyde to aniline in this formaldehyde aqueous solution was 0.33. The addition time was controlled to 25 minutes, with the reaction temperature maintained at 70°C. After the addition was complete, the reaction was maintained at 70°C for 30 minutes.

[0068] (S3) Then, a third portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.06. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 110° C. and the reaction is maintained for 120 minutes to carry out the transposition reaction.

[0069] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0070] In this example, the condensation reaction characteristic index R = 1.029. The DAM obtained according to the above production process had a bicyclic MDA content of 48.2%, an N-methyl impurity content of 0.16%, an aminobenzylamine content of 145 ppm, and a polyamine content of 10 or more rings of 85 ppm. PMDI was prepared by phosgenation of the DAM obtained in accordance with Example 1. After solvent removal and without MDI removal, the PMDI viscosity was measured to be 201 mPa·s (25°C).

[0071] Example 3

[0072] (S1) Mix the first step hydrochloric acid (33% wt) and aniline (99.99% wt), hydrochloric acid (H + The molar ratio of aniline to aniline was 0.12 to obtain aniline hydrochloride reaction solution.

[0073] (S2) Then, a first portion of the formaldehyde aqueous solution (37% wt) was gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction. The molar ratio of formaldehyde to aniline in the first portion of the formaldehyde aqueous solution was 0.25. The addition time of this formaldehyde aqueous solution was controlled to be 20 minutes, and the reaction temperature was maintained at 98°C during the addition.

[0074] When the cumulative molar ratio of formaldehyde to aniline reached 0.12, a second portion of hydrochloric acid was added to the reaction solution. The addition time for this hydrochloric acid addition was controlled to 5 minutes, with a molar ratio of hydrochloric acid to aniline of 0.015. The reaction temperature was maintained at 98°C. After the addition of the second portion of hydrochloric acid was complete, a second portion of formaldehyde aqueous solution was added to the reaction system for condensation reaction. The molar ratio of formaldehyde to aniline in this formaldehyde aqueous solution was 0.08. The addition time was controlled to 25 minutes, with the reaction temperature maintained at 98°C. After the addition was complete, the reaction was maintained at 98°C for 30 minutes.

[0075] (S3) Then, a third portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.015. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 115° C. and the reaction is maintained for 120 minutes to carry out the transposition reaction.

[0076] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0077] In this example, the condensation reaction characteristic index R = 1.023. The DAM obtained according to the above production process had a bicyclic MDA content of 48.7%, an N-methyl impurity content of 0.14%, an aminobenzylamine content of 154 ppm, and a polyamine content of 10 or more rings of 75 ppm. PMDI was prepared by phosgenation of the DAM obtained in accordance with Example 1. After solvent removal and without MDI removal, the PMDI viscosity was measured to be 213 mPa·s (25°C).

[0078] Example 4

[0079] (S1) Mix the first step hydrochloric acid (33% wt) and aniline (99.99% wt), hydrochloric acid (H + The molar ratio of aniline to aniline was 0.224 to obtain aniline hydrochloride reaction solution.

[0080] (S2) Then, a first portion of the formaldehyde aqueous solution (37% wt) is gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction. The molar ratio of formaldehyde to aniline in the first portion of the formaldehyde aqueous solution is 0.10. The addition time of this formaldehyde aqueous solution is controlled to be 20 minutes, and the reaction temperature is maintained at 60°C during this period.

[0081] When the molar ratio of formaldehyde to aniline reached 0.10, a second portion of hydrochloric acid was added to the reaction solution. The addition time for this hydrochloric acid addition was controlled to 5 minutes, with a molar ratio of hydrochloric acid to aniline of 0.014. The reaction temperature was maintained at 60°C. After the addition of the second portion of hydrochloric acid was complete, a second portion of formaldehyde aqueous solution was added to the reaction system for condensation reaction. The molar ratio of formaldehyde to aniline in this formaldehyde aqueous solution was 0.45. The addition time was controlled to 25 minutes, with the reaction temperature maintained at 60°C. After the addition was complete, the reaction was maintained at 60°C for 30 minutes.

[0082] (S3) Then, a third portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.042. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 110° C. and the reaction is maintained for 120 minutes to carry out the transposition reaction.

[0083] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0084] In this example, the condensation reaction characteristic index R = 1.030. The DAM obtained according to the above production process had a bicyclic MDA content of 47.7%, an N-methyl impurity content of 0.17%, an aminobenzylamine content of 134 ppm, and a polyamine content of 10 or more rings of 88 ppm. Referring to Example 1, the DAM obtained was subjected to phosgenation to produce PMDI. After solvent removal and without undergoing the MDI removal step, the PMDI viscosity was measured to be 223 mPa·s (25°C).

[0085] Example 5

[0086] (S1) The first part of hydrochloric acid (33% wt) and aniline (99.99% wt) are mixed, and the hydrochloric acid (as H + The molar ratio of aniline to aniline was 0.18 to obtain aniline hydrochloride reaction solution.

[0087] (S2) Then, a formaldehyde aqueous solution (37% wt) is gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction. The molar ratio of formaldehyde to aniline in the formaldehyde aqueous solution is 0.43. The addition time of the formaldehyde aqueous solution is controlled to be 30 minutes, and the reaction temperature is maintained at 85°C during this period. After the addition is completed, the reaction is continued to be maintained at 85°C for 30 minutes.

[0088] (S3) Then, a second portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.12. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 105° C. and the reaction is maintained for 120 minutes to carry out the transposition reaction.

[0089] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0090] In this example, the condensation reaction characteristic index R = 1.005. The DAM obtained according to the above production process had a bicyclic MDA content of 49.7%, an N-methyl impurity content of 0.20%, an aminobenzylamine content of 102 ppm, and a polyamine content of 10 or more rings of 104 ppm. Referring to Example 1, the DAM obtained was subjected to phosgenation to produce PMDI. After solvent removal and without undergoing the MDI removal step, the PMDI viscosity was measured to be 168 mPa·s (25°C).

[0091] Example 6

[0092] (S1) Mix the first step hydrochloric acid (33% wt) and aniline (99.99% wt), hydrochloric acid (H + The molar ratio of aniline to aniline was 0.15 to obtain aniline hydrochloride reaction solution.

[0093] (S2) Subsequently, a first portion of the formaldehyde aqueous solution (37% wt) is gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction. The molar ratio of formaldehyde to aniline in the first portion of the formaldehyde aqueous solution is 0.35. The addition time of the first portion of the formaldehyde aqueous solution is controlled to be 20 minutes, during which the reaction temperature is maintained at 40°C. After the addition is completed, the reaction is continued at 40°C for 5 minutes. Then, a second portion of the formaldehyde aqueous solution is continuously added to the reaction solution. The molar ratio of formaldehyde to aniline in the second portion of the formaldehyde aqueous solution is 0.24. The addition time is controlled to be 25 minutes, during which the reaction temperature is maintained at 40°C. After the addition is completed, the reaction is continued at 40°C for 30 minutes.

[0094] (S3) Then, a second portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.15. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 105° C. and the reaction is maintained for 120 minutes to carry out the transposition reaction.

[0095] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0096] In this example, the condensation reaction characteristic index R = 1.038. The DAM obtained according to the above production process had a bicyclic MDA content of 47.2%, an N-methyl impurity content of 0.23%, an aminobenzylamine content of 127 ppm, and a polyamine content of 10 or more rings of 112 ppm. Referring to Example 1, the DAM obtained was subjected to phosgenation to produce PMDI. After solvent removal and without MDI removal, the PMDI viscosity was measured to be 219 mPa·s (25°C).

[0097] Example 7

[0098] (S1) Hydrochloric acid (33% wt) and aniline (99.99% wt) were mixed. + The molar ratio of aniline to aniline was 0.25 to obtain aniline hydrochloride reaction solution.

[0099] (S2) Subsequently, a first portion of formaldehyde aqueous solution (37% wt) is gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction, wherein the molar ratio of formaldehyde to aniline in the first portion of formaldehyde aqueous solution is 0.25, and the addition time of the first portion of formaldehyde aqueous solution is controlled to be 20 minutes, during which the reaction temperature is maintained at 98°C. After the addition is completed, the reaction is continued at 98°C for 5 minutes; then, a second portion of formaldehyde aqueous solution is continuously added to the reaction solution, wherein the molar ratio of formaldehyde to aniline in the second portion of formaldehyde aqueous solution is 0.14, and the addition time is controlled to be 25 minutes, during which the reaction temperature is maintained at 98°C. After the addition is completed, the reaction is continued at 98°C for 30 minutes.

[0100] (S3) The reaction temperature is then raised to 120° C. and maintained for 120 min to perform a transposition reaction.

[0101] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0102] In this example, the condensation reaction characteristic index R = 1.034. The DAM obtained according to the above production process had a bicyclic MDA content of 46.3%, an N-methyl impurity content of 0.25%, an aminobenzylamine content of 121 ppm, and a polyamine content of 10 or more rings of 134 ppm. Referring to Example 1, the DAM obtained was subjected to phosgenation to produce PMDI. After solvent removal and without MDI removal, the PMDI viscosity was measured to be 242 mPa·s (25°C).

[0103] Example 8

[0104] (S1) Mix the first step hydrochloric acid (33% wt) and aniline (99.99% wt), hydrochloric acid (H + The molar ratio of aniline to aniline was 0.012 to obtain aniline hydrochloride reaction solution.

[0105] (S2) Subsequently, a first portion of formaldehyde aqueous solution (37% wt) is gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction, wherein the molar ratio of formaldehyde to aniline in the first portion of formaldehyde aqueous solution is 0.30, and the addition time of the first portion of formaldehyde aqueous solution is controlled to be 20 minutes, during which the reaction temperature is maintained at 60°C. After the addition is completed, the reaction is continued at 60°C for 5 minutes; then, a second portion of formaldehyde aqueous solution is continuously added to the reaction solution, wherein the molar ratio of formaldehyde to aniline in the second portion of formaldehyde aqueous solution is 0.19, and the addition time is controlled to be 25 minutes, during which the reaction temperature is maintained at 60°C. After the addition is completed, the reaction is continued at 60°C for 30 minutes.

[0106] (S3) Then, a second portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.228. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 120° C. and the reaction is maintained for 120 minutes to carry out the transposition reaction.

[0107] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0108] In this example, the condensation reaction characteristic index R = 1.022. The DAM obtained according to the above production process had a dicyclic MDA content of 47.7%, an N-methyl impurity content of 0.28%, an aminobenzylamine content of 155 ppm, and a content of ten-ring or higher polyamines of 83 ppm. Referring to Example 1, the DAM obtained was subjected to phosgenation to produce PMDI. After solvent removal and without MDI removal, the PMDI viscosity was measured to be 199 mPa·s (25°C).

[0109] Comparative Example 1

[0110] (S1) Mix the first step hydrochloric acid (33% wt) and aniline (99.99% wt), hydrochloric acid (H + The molar ratio of aniline to aniline was 0.06 to obtain aniline hydrochloride reaction solution.

[0111] (S2) Then, a first portion of the formaldehyde aqueous solution (37% wt) was gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction. The molar ratio of formaldehyde to aniline in the first portion of the formaldehyde aqueous solution was 0.10. The addition time of this formaldehyde aqueous solution was controlled to be 20 minutes, and the reaction temperature was maintained at 70°C during the reaction.

[0112] When the molar ratio of formaldehyde to aniline reached 0.10, a second portion of hydrochloric acid was added to the reaction solution at a molar ratio of 0.02 to aniline, maintaining the reaction temperature at 70°C. After the addition of the second portion of hydrochloric acid was complete, a second portion of aqueous formaldehyde solution was added to the reaction system to initiate a condensation reaction. The molar ratio of formaldehyde to aniline in this aqueous formaldehyde solution was 0.40. The addition time was controlled to 25 minutes, maintaining the reaction temperature at 70°C. After the addition was complete, the reaction was maintained at 70°C for 30 minutes.

[0113] (S3) Then, a third portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.12. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 120° C. and the reaction is maintained for 120 minutes to carry out the transposition reaction.

[0114] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0115] In this example, the condensation reaction characteristic index R = 1.072. The DAM obtained according to the above production process had a dicyclic MDA content of 43.1%, an N-methyl impurity content of 0.28%, an aminobenzylamine content of 163 ppm, and a content of ten-ring or higher polyamines of 172 ppm. Referring to Example 1, the DAM obtained was subjected to phosgenation to prepare PMDI. After solvent removal and without MDI removal, the PMDI viscosity was measured to be 261 mPa·s (25°C).

[0116] Comparative Example 2

[0117] (S1) Mix the first step hydrochloric acid (33% wt) and aniline (99.99% wt), hydrochloric acid (H + The molar ratio of aniline to aniline was 0.02 to obtain aniline hydrochloride reaction solution.

[0118] (S2) Subsequently, a first portion of formaldehyde aqueous solution (37% wt) is gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction, wherein the molar ratio of formaldehyde to aniline in the first portion of formaldehyde aqueous solution is 0.30, and the addition time of the first portion of formaldehyde aqueous solution is controlled to be 20 minutes, during which the reaction temperature is maintained at 65°C. After the addition is completed, the reaction is continued at 65°C for 5 minutes; then, a second portion of formaldehyde aqueous solution is continuously added to the reaction solution, wherein the molar ratio of formaldehyde to aniline in the second portion of formaldehyde aqueous solution is 0.20, and the addition time is controlled to be 25 minutes, during which the reaction temperature is maintained at 65°C. After the addition is completed, the reaction is continued at 65°C for 30 minutes.

[0119] (S3) Then, a second portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.08. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 120° C. and the reaction is maintained for 120 minutes to carry out the transposition reaction.

[0120] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0121] In this example, the condensation reaction characteristic index R = 1.076. The DAM obtained according to the above production process had a dicyclic MDA content of 42.4%, an N-methyl impurity content of 0.26%, an aminobenzylamine content of 235 ppm, and a content of ten-ring or higher polyamines of 168 ppm. Referring to Example 1, the DAM obtained was subjected to phosgenation to prepare PMDI. After solvent removal and without MDI removal, the PMDI viscosity was measured to be 277 mPa·s (25°C).

[0122] Comparative Example 3

[0123] (S1) Mix the first step hydrochloric acid (33% wt) and aniline (99.99% wt), hydrochloric acid (H + The molar ratio of aniline to aniline was 0.175 to obtain aniline hydrochloride reaction solution.

[0124] (S2) Then, a first portion of the formaldehyde aqueous solution (37% wt) was gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction. The molar ratio of formaldehyde to aniline in the first portion of the formaldehyde aqueous solution was 0.10. The addition time of this formaldehyde aqueous solution was controlled to be 20 minutes, and the reaction temperature was maintained at 55°C during the addition.

[0125] When the molar ratio of formaldehyde to aniline reached 0.10, a second portion of hydrochloric acid was added to the reaction solution at a molar ratio of 0.025 to aniline, maintaining the reaction temperature at 55°C. After the addition of the second portion of hydrochloric acid was complete, a second portion of aqueous formaldehyde solution was added to the reaction system to initiate a condensation reaction. The molar ratio of formaldehyde to aniline in this aqueous formaldehyde solution was 0.37. The addition time was controlled to 25 minutes, maintaining the reaction temperature at 55°C. After the addition was complete, the reaction was continued at 55°C for 30 minutes.

[0126] (S3) Then, a third portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.05. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 120° C. and the reaction is maintained for 120 minutes to perform a transposition reaction.

[0127] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0128] In this example, the condensation reaction characteristic index R = 0.956. The DAM obtained according to the above production process had a dicyclic MDA content of 53.3%, an N-methyl impurity content of 0.19%, an aminobenzylamine content of 145 ppm, and a polyamine content of 10 or more rings of 34 ppm. Referring to Example 1, the DAM obtained was subjected to phosgenation to produce PMDI. After solvent removal and without MDI removal, the PMDI viscosity was measured to be 113 mPa·s (25°C).

[0129] Comparative Example 4

[0130] (S1) Mix the first step hydrochloric acid (33% wt) and aniline (99.99% wt), hydrochloric acid (H + The molar ratio of aniline to aniline was 0.052 to obtain aniline hydrochloride reaction solution.

[0131] (S2) Then, a first portion of the formaldehyde aqueous solution (37% wt) was gradually added to the aniline hydrochloride reaction solution obtained in step (S1) to carry out a condensation reaction. The molar ratio of formaldehyde to aniline in the first portion of the formaldehyde aqueous solution was 0.20. The addition time of this formaldehyde aqueous solution was controlled to be 20 minutes, and the reaction temperature was maintained at 50°C during the addition.

[0132] When the cumulative molar ratio of formaldehyde to aniline reached 0.20, a second portion of hydrochloric acid was added to the reaction solution. The addition time for this hydrochloric acid addition was controlled to 5 minutes, with a molar ratio of hydrochloric acid to aniline of 0.026. The reaction temperature was maintained at 50°C. After the addition of the second portion of hydrochloric acid was complete, a second portion of formaldehyde aqueous solution was added to the reaction system for condensation reaction. The molar ratio of formaldehyde to aniline in this formaldehyde aqueous solution was 0.29. The addition time was controlled to 25 minutes, with the reaction temperature maintained at 50°C. After the addition was complete, the reaction was continued at 50°C for 30 minutes.

[0133] (S3) Then, a third portion of hydrochloric acid is added to the condensation reaction solution obtained in step (S2), wherein the molar ratio of the hydrochloric acid to aniline is 0.182. After the addition of the hydrochloric acid is completed, the reaction temperature is raised to 120° C. and the reaction is maintained for 120 minutes to perform a transposition reaction.

[0134] (S4) After the metathesis reaction is completed, a 50% wt aqueous sodium hydroxide solution is added to the metathesis reaction solution at a molar ratio of 1.1 between sodium hydroxide and hydrochloric acid (calculated as HCl). The reaction is maintained at 100°C for 10 minutes. The organic phase is separated and washed with water. The aniline and water are then removed by vacuum distillation at 1 kPa and 220°C to obtain the product DAM.

[0135] In this example, the condensation reaction characteristic index R = 0.984. The DAM obtained according to the above production process had a dicyclic MDA content of 51.5%, an N-methyl impurity content of 0.14%, an aminobenzylamine content of 142 ppm, and a polyamine content of 10 or more rings of 60 ppm. Referring to Example 1, the DAM obtained was subjected to phosgenation to produce PMDI. After solvent removal and without MDI removal, the PMDI viscosity was measured to be 136 mPa·s (25°C).

[0136] The experimental results of each embodiment and comparative example are summarized as follows:

[0137] As can be seen from the above experimental results, in the preparation of DAM in each embodiment, the condensation reaction characteristic index R is controlled to satisfy 1 < R < 1.05. Compared with the comparative examples, DAM with a suitable mass content of diphenylmethane diamine (45%-50%) can be obtained. Finally, PMDI with a suitable viscosity (150-250 mPa·S) can be obtained without additional removal of MDI, and at the same time, the obtained DMA can have a relatively low impurity content. In Comparative Examples 1-2, the condensation reaction characteristic index R ≥ 1.05 during the preparation of DAM. As a result, the mass content of diphenylmethane diamine in the obtained DAM product is too low, the viscosity of the finally obtained PMDI is too high, and the prepared DAM has a significantly high impurity content, especially the contents of aminobenzylamine and polyamines with more than ten rings are significantly higher. In Comparative Examples 3-4, the condensation reaction characteristic index R ≤ 1 during the preparation of DAM. As a result, the mass content of diphenylmethane diamine in the obtained DAM product is too high, and the viscosity of the finally obtained PMDI is significantly low.

[0138] It is easy to understand that the above embodiments are merely examples clearly described and do not mean that the present invention is only limited thereto. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for preparing DAM that is conducive to reducing the content of diphenylmethane diamine, characterized in that, The steps of the described method include: (S1) Aniline reacts with at least part of hydrochloric acid to obtain aniline hydrochloride; (S2) The aniline hydrochloride undergoes a condensation reaction with an aqueous formaldehyde solution to obtain a condensation reaction solution, and the aqueous formaldehyde solution is added once or in multiple portions; optionally, part of hydrochloric acid is added in step (S2); (S3) The condensation reaction solution undergoes a rearrangement reaction to obtain a rearrangement reaction solution; optionally, part of hydrochloric acid is added in step (S3); During the process of performing steps (S1) to (S3), the condensation reaction characteristic index R is controlled to satisfy 1 < R < 1.05, more preferably 1.01 < R < 1.04, where the R is calculated by the following formula (I): R = 0.023(n1 + (-1.158N 2 - 0.025N + 0.528)) × (0.0023T 2 - 0.141T + 45.3) (I) In formula (I), the N is calculated by the following formula (II): N = n2×(a1+(0.32b + 0.64)a2 + 0.64a3) (II) In formula (I), n1 is the molar ratio of the total amount of formaldehyde used to the total amount of aniline used, and T is the reaction temperature in step (S2); in formula (II), n2 is the molar ratio of the total amount of hydrochloric acid used calculated as H + to the total amount of aniline used, a1 is the molar proportion of the amount of hydrochloric acid used calculated as H + in step (S1) in the total amount of hydrochloric acid used, a2 is the molar proportion of the amount of hydrochloric acid used calculated as H + in step (S2) in the total amount of hydrochloric acid used, a3 is the molar proportion of the amount of hydrochloric acid used calculated as H + in step (S3) in the total amount of hydrochloric acid used, and b is the molar proportion of the formaldehyde added for the first time in step (S2) in the total amount of formaldehyde.

2. The method according to claim 1, wherein In step (S2), the aniline hydrochloride is mixed with part of the aqueous formaldehyde solution to perform a first-stage condensation reaction; then, part of the aqueous formaldehyde solution and optionally part of hydrochloric acid are added, and the second-stage condensation reaction is continued to obtain the condensation reaction solution; the temperature of the first-stage condensation reaction and the temperature of the second-stage condensation reaction are the same or different. When the temperature of the first-stage condensation reaction and the temperature of the second-stage condensation reaction are different, the value of T in formula (I) is the logarithmic average of the condensation reaction temperatures of each stage; In step (S3), the condensation reaction solution is mixed with part of hydrochloric acid, and then the rearrangement reaction is carried out.

3. The method according to claim 1 or 2, characterized in that, The described preparation method further includes the following steps: (S4) The rearrangement reaction solution is neutralized, then phase-separated to obtain an organic phase, and the organic phase is refined after washing with water to obtain the DAM.

4. The method according to any one of claims 1 to 3, characterized in that, In step (S2), the reaction temperature T is 40 - 100 °C, preferably 45 - 95 °C.

5. The method according to any one of claims 1-4, characterized in that, The molar ratio n1 of the total amount of formaldehyde used to the total amount of aniline used is 0.2 - 0.6, preferably 0.3 - 0.

55.

6. The method according to any one of claims 1-5, characterized in that, The total amount of hydrochloric acid and the total amount of aniline are in a molar ratio n2 of 0.01 - 0.4, preferably 0.05 - 0.

3. + The total amount of hydrochloric acid and the total amount of aniline are in a molar ratio n2 of 0.01 - 0.4, preferably 0.05 - 0.

3.

7. The method according to any one of claims 1-6, characterized in that, The a1, a2, and a3 respectively satisfy the following requirements: 0 < a1 ≤ 1, 0 ≤ a2 < 1, 0 ≤ a3 < 1, and a1 + a2 + a3 = 1; Preferably, a1 is 0.1 - 1, a2 is 0 - 0.4, and a3 is 0 - 0.

8.

8. The method according to any one of claims 1-7, characterized in that, In step (S2), the molar proportion b of the formaldehyde added for the first time to the total amount of formaldehyde is 0.01 - 1, preferably 0.01 - 0.99, more preferably 0.2 - 0.8; Preferably, the number of times of adding the aqueous formaldehyde solution in step (S2) is 2 - 5 times.

9. The method according to any one of claims 1-8, characterized in that, The mass content of diphenylmethane diamine in the DAM is 45% - 50%.

10. A method for preparing PMDI, in which DAM undergoes a phosgenation reaction with phosgene, and the solvent is removed to obtain the PMDI, characterized in that, The DAM is the DAM prepared by the method according to any one of claims 1 - 10, and after completing the phosgenation reaction, no MDI removal operation is required; Preferably, the viscosity of the PMDI at 25 °C is 150 - 250 mPa·S.

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