Polyisocyanate and preparation method thereof

Abstract

The invention discloses a preparation method of polyisocyanate. The preparation method comprises the following steps: (1) polymerizing an isocyanate raw material under the action of a catalyst; (2) after the reaction reaches a set conversion rate, adding a catalyst poison to terminate the reaction; (3) separating and removing isocyanate monomers which do not participate in the reaction to obtain a polyisocyanate product; wherein, based on the weight of the catalyst poison, the content of the inorganic salt in the catalyst poison is 10 to 1000 ppm. The prepared polyisocyanate composition does not precipitate crystals after being stored at the temperature of-20 DEG C or below for 3 months.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparing isocyanate derivatives, and particularly relates to a preparation method of polyisocyanate. Background Art

[0002] Polyisocyanates prepared by polymerizing aliphatic diisocyanate monomers have been widely used in the polyurethane coating or adhesive industry. By self-polymerization of isocyanates, the hazards of volatile isocyanate monomers can be effectively reduced. Isocyanate monomers can obtain structures such as uretidione, trimer, and iminooxadiazinedione through self-polymerization. Isocyanate homopolymers containing uretidione groups have very low viscosities. Therefore, they have excellent application performance as cross-linking agent components in waterborne, low-solvent, and high-solids coating compositions. Especially as a diluent for curing agents to replace solvents, their preparation methods have been reported in many patents, and the catalysts used mainly include tertiary phosphines, triazole salt compounds, antimony fluoride, sulfamide salts, etc., among which tertiary phosphines are commonly used catalysts.

[0003] For example, GB115385 and GB1145952 describe the preparation of uretidione polyisocyanates using tributylphosphine as a catalyst, and US6043332 discloses the use of trisubstituted phosphite stabilizers to improve product stability. CN105189597 mentions that moisture in catalyst poisons has an adverse effect on the product process, and WO2022061704 improves product stability by heating at a temperature 10 - 30°C higher than the thermal sensitivity temperature for 5 - 30 minutes.

[0004] In practical applications, during the production process using tertiary phosphine as a catalyst, due to its active physical properties, it is easy to react with other chemical substances, and impurities in catalyst poisons will affect the stability of the product, resulting in unstable phenomena during storage, such as crystallization and precipitation. Therefore, how to control the stability of polyisocyanate compositions during storage is a crucial factor. Summary of the Invention

[0005] The purpose of the present invention is to provide a polyisocyanate and its preparation method. The product prepared by this method will not show crystallization and precipitation phenomena during storage below -20°C, meeting the downstream application requirements.

[0006] In order to achieve the purpose of the present invention, the following technical solutions are adopted:

[0007] In the first aspect of the present invention, a preparation method of polyisocyanate is first provided, including the following steps:

[0008] (1) Polymerize organic isocyanate raw materials under the action of a catalyst;

[0009] (2) After the reaction reaches the set conversion rate, a catalyst poison is added to terminate the reaction;

[0010] (3) An appropriate separation method is used to remove the unreacted organic isocyanate monomer to obtain a product containing polyisocyanate;

[0011] Among them, the inorganic salt in the catalyst poison is controlled to be 10-1000 ppm based on the weight of the catalyst poison.

[0012] We surprisingly found that the inorganic salt in the catalyst poison in the reaction system for preparing polyisocyanate is difficult to remove by separation, is hardly soluble in the product, and will induce the crystallization and precipitation of the termination product. Especially in a low-temperature environment, this precipitation process will be significantly accelerated. The inventor can greatly improve the storage stability of the product by controlling the content of the inorganic salt in the catalyst poison. When the inorganic salt in the catalyst poison is controlled to be 10-1000 ppm based on the weight of the catalyst poison, the separated product is stored at -20 °C or below for 3 months, the appearance of the product is uniform, the viscosity change of the product during storage is ≤10%, and no precipitate is generated. Based on this, we completed the present invention.

[0013] In the method of the present invention, the preparation method of the isocyanate raw material can adopt existing conventional methods, including isocyanate monomers produced by using phosgene, not using phosgene or any other method, such as aromatic, aliphatic, and / or alicyclic organic isocyanates.

[0014] In the method of the present invention, the aromatic, aliphatic, and / or alicyclic organic isocyanates are organic diisocyanates or polyisocyanates containing 4-20 carbon atoms in the carbon skeleton in addition to the NCO group. The isocyanates are selected from one or more of hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), cyclohexyl dimethylene diisocyanate (HMDI), xylylene diisocyanate (XDI), dicyclohexylmethane diisocyanate (HXDI), norbornane dimethylene diisocyanate (NBDI), cyclohexyl diisocyanate (CHDI), lysine diisocyanate (LDI), tetramethylxylylene diisocyanate (TMXDI), 2,4,4-trimethylhexane diisocyanate (TMHDI), toluene diisocyanate (TDI), methylcyclohexyl diisocyanate (HTDI), naphthalene diisocyanate (NDI), p-phenylene diisocyanate (PPDI), diphenylmethane diisocyanate (MDI), polymethylene polyphenyl polyisocyanate (PM), etc.

[0015] In the present invention, the catalyst is a tertiary phosphine catalyst selected from trialkyl phosphines such as trimethyl phosphine, triethyl phosphine, tripropyl phosphine, triisopropyl phosphine, tri-n-butyl phosphine, tri-tert-butyl phosphine, dicyclopentylbutyl phosphine, tripentyl phosphine, tricyclopentyl phosphine, trihexyl phosphine, tricyclohexyl phosphine, tri-n-octyl phosphine, etc. The more preferred catalyst is tributyl phosphine and / or trioctyl phosphine.

[0016] The amount of the catalyst used is generally 0.01-1 wt%, preferably 0.05-0.5 wt%, based on the amount of the starting isocyanate used.

[0017] In the present invention, the catalyst tertiary phosphine is a nucleophilic reagent, which is easily oxidized by oxygen in the air. It must be strictly deoxygenated when used and protected by an inert gas. For example, if trioctyl phosphine is exposed to air, a violent oxidation reaction will occur to generate trioctyl phosphine oxide.

[0018] In the present invention, a suitable alcohol may be optionally selected as a co-catalyst for use with the tertiary phosphine catalyst, and the co-catalyst includes a low molecular weight monovalent or polyvalent aliphatic alcohol, preferably an alcohol with a molecular weight of 32 to 250. These alcohols include, for example, one or more of diols or polyols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, n-hexanol, octanol, 2-ethyl-1-hexanol, ethylene glycol, propylene glycol, isomeric butanediols, pentanediol, neopentyl glycol, hexanediol, octanediol, diethylene glycol, dipropylene glycol, 2-ethyl-1,3-hexanediol, 2,2,4-dimethylpentanediol, glycerol, and trimethylolpropane.

[0019] The amount of the cocatalyst is 0-5%, preferably 0.1-5%, more preferably 0.5-3% of the weight of the starting isocyanate used. When a cocatalyst is used, the substance that actually acts as a cocatalyst is the carbamate formed by the reaction of the cocatalyst and the starting isocyanate. Therefore, it is also appropriate to prepare the alcohol separately by reacting the alcohol with the isocyanate and then adding it to the reaction in the form of a cocatalyst instead of using the alcohol.

[0020] In step (1), the polymerization reaction temperature is 30-150°C.

[0021] In the method of the present invention, when the ratio of the consumed mass of isocyanate in the system to the total mass of isocyanate monomers in the system reaches 10%-80%, preferably 30%-70%, a terminator (catalyst poison) is added to terminate the reaction. During the termination process, the reaction mixture needs to be heated to above 80°C, but not higher than 150°C, and maintained for more than 0.5 hours, preferably for more than 2 hours.

[0022] The catalyst poisons used are: acyl chlorides (such as formyl chloride, acetyl chloride, benzoyl chloride or phthaloyl chloride, etc.), sulfonic acid esters (such as methyl p-toluenesulfonate and ethyl p-toluenesulfonate, etc.), alkyl phosphates (such as monobutyl phosphate, dibutyl phosphate, monoethyl phosphate, diethyl phosphate, dioctyl phosphate, diisooctyl phosphate, etc.), and sulfate esters (such as dimethyl sulfate, diethyl sulfate, etc.), one or more of them. The dosage of the catalyst poison is 80-120% of the molar amount of the catalyst used in the reaction.

[0023] During the research process, the applicant found that the catalyst poison is a key factor affecting the storage stability of the product. That is, the inorganic salts in the catalyst poison enter the uretdione polyisocyanate product after separating the unreacted isocyanate monomer. Since these inorganic salt substances are difficult to dissolve in the isocyanate, they induce the formation of a combined product of the termination catalyst and the catalyst poison during the storage of the uretdione polyisocyanate. Especially in a low-temperature storage environment, this inducing effect is more obvious, which further leads to a phenomenon similar to crystal precipitation in the product. The research found that when the inorganic salts in the catalyst poison are controlled at 10-1000 ppm, preferably 100-500 ppm based on the weight of the catalyst poison, the separated product can be stored at below -20 °C for 3 months without crystal precipitation, and the viscosity change of the product ≤ 10%.

[0024] The inorganic salts in the catalyst poison are raw materials during production or impurities remaining in the product, including sodium chloride, potassium chloride, sodium sulfonate, sodium sulfate, sodium phosphate, etc.; these inorganic salts can be controlled within the range required by the present invention through conventional separation means, including rectification, crystallization, membrane separation and other means.

[0025] The method of the present invention can be implemented without a solvent, or can be implemented in the presence of a solvent inert to the isocyanate. Suitable solvents are such as: butyl acetate, ethyl acetate, tetrahydrofuran, propylene glycol methyl ether acetate, xylene, propylene glycol diacetate, methyl ethyl ketone, methyl isoamyl ketone, cyclohexanone, hexane, toluene, xylene, benzene, chlorobenzene, o-dichlorobenzene, hydrocarbon mixture, dichloromethane, etc. However, the reaction of the present invention is preferably carried out without adding a solvent.

[0026] In the method of the present invention, the starting isocyanate monomer is heated to 30-150 °C in an inert gas (such as nitrogen) atmosphere and in the presence of an optional above-mentioned solvent. Then, the tertiary phosphine catalyst and an optional cocatalyst are added to the reaction system in the amounts given above, and the reaction temperature is optionally controlled at 30-150 °C using an appropriate method (heating, cooling). In the method of the present invention, the alcohol cocatalyst can be added to the reaction system at any reaction stage. For example, the cocatalyst can be added to the starting isocyanate monomer before the reaction, or can be added after the catalyst is added, or can be added after the reaction reaches a certain conversion rate.

[0027] The catalyst in the present invention can be used without dilution or in the form of a solution in a solvent. Suitable solvents include all compounds that do not react with phosphine, such as aliphatic or aromatic hydrocarbons, alcohols, ketones, esters, and ethers. In the method of the present invention, it is preferred to use an alcohol or not to dilute the catalyst with a solvent.

[0028] In the method of the present invention, the appropriate separation method described in step (3) is a combination of one or more of a flash evaporator, a falling film evaporator, a thin film evaporator, and a short path evaporator. Since the uretdione polyisocyanate is prone to decomposition at high temperatures, in order to reduce the decomposition of the uretdione polyisocyanate, the separation temperature needs to be controlled at 100 - 160 °C, preferably 110 - 150 °C, and the residence time of the material at high temperature is 2 min to 30 min, preferably 5 min to 20 min.

[0029] The unreacted isocyanate monomer obtained by separation and the solvent that may be added can be returned to the reaction kettle to participate in the reaction, or can participate in the reaction after distillation or rectification treatment. In the method of the present invention, it is preferred to directly return to the reaction kettle to participate in the reaction without distillation or rectification treatment.

[0030] In addition, stabilizers and additives can be added anywhere in the method of the present invention, and these stabilizers and additives are conventional in the field of polyisocyanates. Including but not limited to: antioxidants, sterically hindered phenols (such as 2,6 - di - tert - butyl - 4 - methylphenol, octadecyl 3,5 - di - tert - butyl - 4 - hydroxyphenylpropionate, etc.), phosphites (such as tris(nonylphenyl) phosphite, tris(2,4 - di - tert - butylphenyl) phosphite, etc.), ultraviolet absorbers (such as benzotriazoles, salicylates, benzophenones, etc.), hindered amine light stabilizers (such as 2,2,6,6 - tetramethylpiperidine), etc.

[0031] In a second aspect, the present invention provides a polyisocyanate product containing a uretdione group prepared by the above method.

[0032] In a third aspect of the present invention, there is provided the use of a polyisocyanate product containing a uretdione group prepared by the method of the present invention for preparing one - component and multi - component polyurethane coatings or adhesives, which can be used together with di - or polyisocyanate products prepared by the prior art, such as mixtures of di - or polyisocyanates containing biuret, urethane, urethane - urea, isocyanurate, and iminooxadiazinedione.

[0033] The present invention also relates to related products such as polyurethane coatings and polyurethane adhesives prepared from the polyisocyanate containing a uretdione group prepared by the method of the present invention. In addition, the uretdione polyisocyanate prepared by the method of the present invention can be used to prepare other related products such as polyurethane coatings and polyurethane adhesives after being blocked with a blocking agent.

[0034] Compared with the prior art, the present invention has the following advantages:

[0035] Uretdione polyisocyanate has a tendency to decompose into free isocyanate monomers during storage. The present invention can greatly improve the storage stability of the product by controlling the content of inorganic salts in the catalyst poison. When the weight of inorganic salts in the catalyst poison is controlled at 10 - 1000 ppm, the obtained uretdione polyisocyanate product is stored at -20 °C or below for 3 months, without crystal precipitation, and the viscosity change rate of the product ≤ 10%. Detailed implementation manners

[0036] The following examples will further illustrate the method provided by the present invention. However, the present invention is not limited to the listed examples, and should also include any other well-known changes within the scope of the claims of the present invention. The specific application of the present invention is not limited to the applications mentioned in the examples. Simple changes made to the present invention by those familiar with the art using the concept of the present invention are within the scope of protection of the present invention.

[0037] The following test methods are adopted in the examples of the present invention:

[0038] (1) Determination of reaction conversion rate:

[0039] Gel chromatography technology (LC-20AD / RID-10A, chromatographic column: MZ-Gel SD plus10E3A, 5μm (8.0 * 300mm), MZ-Gel SDplus 500A 5μm (8.0 * 300mm), MZ-Gel SDplus 100A5μm (8.0 * 300mm) in series, Shimadzu; mobile phase: tetrahydrofuran; flow rate: 1.0 mL / min; analysis time: 40 min, chromatographic column temperature: 35 °C) is used for quantitative determination of isocyanate raw materials. The areas of polymers and monomers in the system are determined by the area normalization method. Reaction conversion rate (%) = 1 - S(monomer peak area) / S(sum of peak areas of each component) * 100%.

[0040] (2) NCO content test is carried out according to standard GB / T 12009.4;

[0041] (3) Viscosity determination method: Dynamic mechanical viscosity is measured using a Brook Field DV-IPrime viscometer with an S21 rotor at 25 °C.

[0042] (4) Test method for inorganic salt content:

[0043] Add m g of the sample (weigh between 1 - 2 g, accurate to 0.0001 g) to a porcelain crucible m1 that has already been weighed to a constant weight. Burn it in a high-temperature muffle furnace at a temperature of 700 - 800 °C until it is completely ashed. Transfer it to a desiccator and wait until it has completely cooled before weighing the total weight again as m2 g.

[0044] Inorganic salt content = (m2 - m1) / m.

[0045] Unless otherwise specified, during the process from before the reaction to the addition of the catalyst and throughout the entire reaction, the reaction solution is kept under the protection of dry nitrogen. Unless otherwise stated, all percentages are by mass.

[0046] Example 1

[0047] For hexamethylene diisocyanate (abbreviated as HDI) with a total mass M of 1000 g, while stirring at 50 °C, sequentially add 15 g of 2-ethyl-1,3-hexanediol and 2.5 g of tri-n-octylphosphine, and quantitatively monitor the proportion of the consumed mass M1 of HDI in the reaction system to the total mass M of the added HDI through gel chromatography; when the consumed mass M1 of HDI in the system accounts for 40% of the total mass M of the added HDI, add 2.2 g of diisooctyl phosphate and heat to 90 °C to maintain the reaction for 2 hours until it terminates. The inorganic salt content in the diisooctyl phosphate is 200 ppm. Carry out two-stage thin-film evaporator distillation at a temperature of 140 °C and a pressure of 0.3 mbar to remove the unreacted HDI in the reaction system, obtaining a polyisocyanate product containing uretdione groups. After testing, the performance and group content of the polyisocyanate product obtained in Example 1 are as follows:

[0048] Viscosity: 156 mPas / 25 °C;

[0049] Content of NCO groups: 21.75%;

[0050] Comparative Example 1

[0051] For comparison, prepare HDI uretdione polyisocyanate using a similar method. Add inorganic salt (NaCl) to diisooctyl phosphate to reach a content of 1000 ppm, and keep other reaction conditions and separation methods the same as in Example 1. After testing, the performance and group content of the obtained polyisocyanate product are as follows:

[0052] Viscosity: 163 mPas / 25 °C;

[0053] Content of NCO groups: 21.72%;

[0054] Then store the two uretdione polyisocyanates at -20 °C, and measure the state and viscosity of each product every month. The results are shown in Table 1 below:

[0055] Table 1

[0056]

[0057]

[0058] It can be seen by comparison that according to Example 1 of the present invention, by controlling the content of inorganic salts in the catalyst poison added during the preparation of the uretdione polyisocyanate, although the separation conditions are the same and the isocyanate values are similar, after storing for 2 months at low temperature in Comparative Example 1, the apparent state begins to show abnormalities, and the sample begins to show signs of opacity. After three months, precipitation occurs in the sample, and the viscosity of the sample increases to 194 mPas, with an increase of 19%.

[0059] Examples 2-7 and Comparative Examples 2-3

[0060] The uretdione polyisocyanate was prepared by the method described in Example 1, and HDI uretdione polyisocyanate was prepared from HDI. In order to compare the effects of different inorganic salts in different catalyst poisons, the reaction process conditions, separation methods and conditions, types and dosages of catalysts and cocatalysts were all the same. The only difference was the catalyst poison and the difference in the content of inorganic salts in the poison. Then, the prepared uretdione polyisocyanate was stored at -20°C. After 3 months, the appearance of each product was observed respectively. The results are shown in Table 2 below:

[0061] Table 2

[0062]

[0063]

[0064] It can be seen by comparison that according to Examples 2-7 of the present invention and Comparative Examples 2-3, by controlling the content of inorganic salts in the catalyst poison added during the preparation of the uretdione polyisocyanate, although the initial states are not much different, after storing for 3 months in an environment of -20°C, there is an obvious precipitation phenomenon in the products with a high content of inorganic salts in the catalyst poison during storage.

[0065] Example 8

[0066] 1000 g of hexamethylene diisocyanate (abbreviation: HDI) with a total mass M was added with 4 g of 2-ethyl-1,3-hexanediol and 3 g of tributylphosphine successively under stirring at 50°C, and the proportion of the consumed mass M1 of HDI in the reaction system to the total mass M of the added HDI was quantitatively monitored by gel chromatography;

[0067] When the consumed mass M1 of HDI in the system accounts for 50% of the total mass M of the added HDI, an equimolar amount of methyl p-toluenesulfonate is added and heated to 150 °C for 0.5 hours to terminate the reaction. The inorganic salt content in methyl p-toluenesulfonate is 300 ppm. Distillation is carried out in a two-stage thin-film evaporator at a temperature of 140 °C and a pressure of 0.3 mbar to remove the unreacted HDI in the reaction system, and a polyisocyanate product containing uretdione groups is obtained. After testing, the performance and group content of the polyisocyanate product obtained in Example 8 are as follows:

[0068] Viscosity: 172 mPas / 25 °C;

[0069] Content of NCO groups: 21.65%.

[0070] Comparative Example 4

[0071] For comparison, HDI uretdione polyisocyanate was prepared by a similar method. However, the inorganic salt content of dimethyl sulfate in the catalyst poison was 2000 ppm, and other reaction conditions and separation methods were the same as those in Example 8. After testing, the performance and group content of the obtained polyisocyanate product are as follows:

[0072] Viscosity: 174 mPas / 25 °C;

[0073] Content of NCO groups: 21.62%;

[0074] Then, the two uretdione polyisocyanates were stored at -20 °C for 3 months, and their appearances were examined. The results are shown in Table 3 below:

[0075] Table 3

[0076]

[0077] Example 9

[0078] The catalyst poison used in Comparative Example 4 was treated as follows to reduce the inorganic salt content in the system. 50 g of dimethyl sulfate was taken, 100 g of deionized water was added, and they were mixed at room temperature for 30 min. After standing and separating, the lower layer material was taken for analysis, and the inorganic salt content in the system was 20 ppm;

[0079] In addition, HDI uretdione polyisocyanate was prepared by a similar method. The reaction conditions and separation methods were the same as those in Example 5, except that the treated dimethyl sulfate was used. The product data are as follows:

[0080] Viscosity: 172 mPas / 25 °C;

[0081] Content of NCO groups: 21.68%;

[0082] After the product is stored at -20°C for 3 months, the samples are homogeneous and the viscosity is 178 mPas / 25°C.

[0083] Comparative Example 5

[0084] The catalyst poison used in Comparative Example 4 was treated as follows to reduce the content of inorganic salts in the system. 100 g of dimethyl sulfate was taken and subjected to vacuum distillation. The fraction with a pressure of 2 KPa and a temperature of 90 - 95°C was collected. After treatment, the content of inorganic salts in dimethyl sulfate was 5 ppm.

[0085] In addition, HDI uretdione polyisocyanate was prepared by a similar method. The reaction conditions and separation methods were the same as those in Example 5, except that the treated dimethyl sulfate was used. The product data is as follows:

[0086] Viscosity: 172 mPas / 25°C;

[0087] Content of NCO groups: 21.68%;

[0088] After the product is stored at -20°C for 3 months, the samples are homogeneous and the viscosity is 206 cp / 25°C. Although the apparent state is stable during storage, the product viscosity increases by about 20%, and the product stability is still poor.

Claims

1. A method for preparing a polyisocyanate, comprising the following steps: (1) The isocyanate raw material is polymerized under the action of a catalyst; (2) After the reaction reaches the set conversion rate, a catalyst poison is added to terminate the reaction; (3) The unreacted isocyanate monomer is separated and removed to obtain a polyisocyanate product; Among them, based on the weight of the catalyst poison, the content of inorganic salts in the catalyst poison is 10 - 1000 ppm, preferably 100 - 500 ppm.

2. The preparation method according to claim 1, characterized in that The reaction temperature of the polymerization is 30 - 150 °C.

3. The preparation method according to claim 1 or 2, characterized in that The isocyanate is selected from one or more of hexamethylene diisocyanate, isophorone diisocyanate, cyclohexyl dimethylene diisocyanate, benzene dimethylene diisocyanate, dicyclohexylmethane diisocyanate, norbornane dimethylene diisocyanate, cyclohexyl diisocyanate, lysine diisocyanate, tetramethylbenzene dimethylene diisocyanate, 2,4,4 - trimethylhexane diisocyanate, toluene diisocyanate, methylcyclohexyl diisocyanate, naphthalene diisocyanate, p - phenylene diisocyanate, diphenylmethane diisocyanate, polymethylene polyphenylene polyisocyanate, preferably hexamethylene diisocyanate.

4. The preparation method according to any one of claims 1 - 3, characterized in that The catalyst is a tertiary phosphine catalyst, preferably a trialkyl phosphine.

5. The preparation method according to claim 4, characterized in that The tertiary phosphine catalyst is tributyl phosphine or trioctyl phosphine.

6. The preparation method according to any one of claims 1 - 5, characterized in that The dosage of the catalyst is 0.01 - 1 wt% of the amount of the starting isocyanate, preferably 0.05 - 0.5 wt%.

7. The preparation method according to any one of claims 1 - 6, characterized in that The catalyst poison includes one or more of acyl chlorides, sulfonic acid esters, alkyl phosphates, and sulfates. Preferably, the acyl chlorides include one or more of formyl chloride, acetyl chloride, benzoyl chloride, and phthaloyl chloride; the sulfonic acid esters include one or more of methyl p - toluenesulfonate and ethyl p - toluenesulfonate; the alkyl phosphates include one or more of monobutyl phosphate, dibutyl phosphate, monoethyl phosphate, diethyl phosphate, dioctyl phosphate, and diisooctyl phosphate; the sulfates include one or more of dimethyl sulfate and diethyl sulfate; And / or, the dosage of the catalyst poison is 80 - 120% of the molar amount of the catalyst used.

8. The preparation method according to any one of claims 1 - 7, characterized in that When the ratio of the consumed mass of the isocyanate in the system to the total mass of the isocyanate reaches 10% - 80%, preferably 30% - 70%, the reaction is terminated; preferably, during the termination process, the reaction mixture needs to be heated to above 80 °C but not higher than 150 °C and maintained for more than 0.5 hours, more preferably maintained for more than 2 hours.

9. The preparation method according to any one of claims 1 - 8, characterized in that The separation method in step (3) is a combination of one or more of a flash evaporator, a falling - film evaporator, a thin - film evaporator, and a short - path evaporator; the separation temperature is 100 - 160 °C, and the residence time of the material is 2 min to 30 min.

10. The polyisocyanate prepared by the method according to any one of claims 1 - 9, characterized in that There is no floc precipitation after storage at - 20 °C for 3 months.

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