JP2003171435A - Method for producing allophanate-modified polyisocyanate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a low-viscosity allophanate- modified polyisocyanate soluble to poor solvent and having sufficiently high isocyanate content. <P>SOLUTION: This method for producing the allophanate-modified polyisocyanate, substantially free from any organic solvent and urethane group, etc., ≤2,000 Pa-s in viscosity at 25°C and 5-25 mass% in NCO content, comprises the 1st step of charging (a) an organic diisocyanate, (b) a PPG-based polyol with f=1 to 3 and Mn=500 to 3,000 and (c) a 1-20C alkyl alcohol so as to be >1 in the molar ratio NCO/OH and then making an allophanate-modifying reaction at 70-150°C in the presence of (d) a zirconium carboxylate until the urethane groups are substantially gone, the 2nd step of discontinuing the allophanate-modifying reaction by adding (e) a catalytic poison, and the 3rd step of removing the component(a) until its content comes to ≤1 mass%. <P>COPYRIGHT: (C)2003,JPO

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing an allophanate-modified polyisocyanate. More specifically, the present invention relates to a method for producing an allophanate-modified polyisocyanate having low viscosity, good solubility in a poor solvent, and excellent durability. [0002] Polyisocyanate curing agents are widely used as curing agents for paints, adhesives and the like. Among them, polyisocyanates having an isocyanurate group are known to be excellent in weather resistance, heat resistance, and durability. [0003] However, polyisocyanates having isocyanurate groups cannot be said to have high solubility and often have high viscosity. For this reason, when used as a curing agent, it is necessary to dilute with an organic solvent such as ethyl acetate, butyl acetate, and methyl ethyl ketone, which have strong dissolving power and are highly toxic. Such an organic solvent has a risk of an odor problem and erosion of a base. In order to further improve the workability, it is necessary to lower the solid content. In order to solve such a problem, it has been proposed to use an allophanate group-containing polyisocyanate. For example, JP-A-8-325516 proposes a polyisocyanate having an isocyanurate group and an allophanate group. In addition, Japanese Patent Application Laid-Open
JP-168155A proposes a polyisocyanate having an allophanate group obtained from a polyol having a specific molecular weight and an organic diisocyanate. [0005] However, the polyisocyanate actually produced in JP-A-8-325516 has an isocyanurate group and therefore has too high a viscosity. In addition, Japanese Patent Application Laid-Open No. 10-1
In Japanese Patent No. 68155, since the molecular weight of the polyol is large, the isocyanate content of the obtained polyisocyanate is low. In view of the above problems, an object of the present invention is to provide a method for producing an allophanate-modified polyisocyanate which has a low viscosity, is soluble in a poor solvent, and has a sufficient isocyanate content. Means for Solving the Problems As a result of intensive studies, the present inventors have found that allophanate group-containing polyisocyanates obtained by a production method using a specific raw material and a specific catalyst are:
The inventors have found that the above-mentioned problems have been solved, and have completed the present invention. That is, the present invention contains substantially no organic solvent, urethane group, uretdione group or isocyanurate group, has a viscosity at 25 ° C. of 2,000 mPa · s or less, and has an isocyanate content of 5 to 25% by mass. A production method characterized in that the allophanate-modified polyisocyanate is performed in the following steps. First step: an organic diisocyanate (a), a polypropylene glycol-based polyether polyol having a nominal number of functional groups of 1 to 3 and a number average molecular weight of 500 to 3,000 (hereinafter referred to as a PPG-based polyol) (b), and a carbon number of 1 To 20 to 20 alkyl alcohols (c) in an amount such that the isocyanate groups become excessive with respect to the hydroxyl groups.
A step of performing an allophanate reaction at 150 ° C. in the presence of a zirconium carboxylate (d) as an allophanate catalyst until substantially no urethane group is present. Second step: a step of adding a catalyst poison (e) to stop the allophanation reaction. Third step: a step of removing free organic diisocyanate (a) until the content becomes 1% or less. The raw materials used in the present invention will be described. As the organic diisocyanate (a) used in the present invention, 2,4-tolylene diisocyanate,
2,6-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 2,2'-diphenylmethane diisocyanate, 4,4'-diphenyl ether diisocyanate, 2-nitrodiphenyl-4,4'- Diisocyanate, 2,2'-diphenylpropane-4,4 '
-Diisocyanate, 3,3'-dimethyldiphenylmethane-4,4'-diisocyanate, 4,4'-diphenylpropane diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, naphthylene-1,4-diisocyanate, naphthylene-1,5
Aromatic isocyanates such as diisocyanate, 3,3'-dimethoxydiphenyl-4,4'-diisocyanate; hexamethylene diisocyanate (hereinafter referred to as HDI), 1,4-tetramethylene diisocyanate;
Aliphatic isocyanates such as 2-methylpentane-1,5-diisocyanate and lysine diisocyanate; xylylene-1,4-diisocyanate, xylylene-1,3
Araliphatic diisocyanates such as diisocyanates;
Examples include alicyclic diisocyanates such as isophorone diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylene diisocyanate, hydrogenated diphenylmethane diisocyanate, and hydrogenated tetramethylxylene diisocyanate. These organic diisocyanates may be used alone or in the form of a mixture of two or more. [0010] Among the above-mentioned organic diisocyanates,
Considering the weather resistance and the like, HDI, which is a non-yellowing isocyanate, is preferred. The PPG polyol (b) used in the present invention has a nominal number of functional groups of 1 to 3 and a number average molecular weight of 500.
3,000 hydroxyl group-containing polyethers, preferably having a nominal number of functional groups of 1-2, and a number average molecular weight of 500-2,
000 polyethers. If the number average molecular weight is too large, the isocyanate content of the resulting polyisocyanate will be small, and if this is used as a curing agent for coatings, the physical properties of the coating film will not be easily exhibited. On the other hand, if the number average molecular weight is too small, the resulting polyisocyanate becomes difficult to dissolve in the poor solvent. If the nominal number of functional groups is too high,
The polyisocyanate obtained may have a high number of functional groups and the viscosity may be too high. The “polypropylene” means that the main repeating structural unit is an oxypropylene group, and means that it may contain another oxyalkylene group such as an oxyethylene group. (B) is obtained by a known method, for example, monools such as methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol and t-butanol, ethylene glycol, 1, 2-
Propanediol, 1,3-propanediol, 1,2
-Butanediol, 1,3-butanediol, 1,4-
Butanediol, 1,5-pentanediol, 2-methyl-1,5-pentanediol, 3-methyl-1,5-
Pentanediol, 1,6-hexanediol, neopentyl glycol, 1,8-octanediol, 1,9
-Nonanediol, 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, -Ethyl-1,3-hexanediol, 2-n-hexadecane-1,2-ethylene glycol, 2-n-eicosan-1,2-ethylene glycol, 2-n-octacosan-1,2-ethylene glycol, diethylene glycol , Dipropylene glycol,
1,4-cyclohexanedimethanol, or an ethylene oxide or propylene oxide adduct of bisphenol A, hydrogenated bisphenol A, 3-hydroxy-2,2-dimethylpropyl-3-hydroxy-2,
Diols such as 2-dimethylpropionate; triols such as trimethylolpropane and glycerin; monoamines such as ethylamine and aniline; monohydroxymonoamino-type amino alcohols such as monoethanolamine; Ring-opening polymerization of an alkylene oxide containing propylene oxide as a main component using two or more initiators is exemplified. In addition,
(B) may be used alone or in combination of two or more. The preferred (b) in the present invention is that, in consideration of the solubility of the obtained polyisocyanate in a poor solvent, the oxypropylene group in the repeating structural unit is 80 mol%.
That is all. The alkyl alcohol (c) used in the present invention is a compound having 1 to 20 carbon atoms, preferably 3 to 10 carbon atoms. (C) includes methanol, ethanol,
Examples thereof include propanol, butanol, pentanol, hexanol, octanol, and stearyl alcohol. In addition, (c) can be used alone or in combination of two or more. The present invention is characterized in that a zirconium carboxylate (d) is used as the allophanate-forming catalyst. The carboxylic acids include acetic acid, propionic acid,
Saturated aliphatic carboxylic acids such as butyric acid, caproic acid, octylic acid, lauric acid, myristic acid, palmitic acid, stearic acid and 2-ethylhexanoic acid; saturated monocyclic carboxylic acids such as cyclohexanecarboxylic acid and cyclopentanecarboxylic acid; bicyclo (4.4.0) saturated bicyclic carboxylic acids such as decane-2-carboxylic acid, mixtures of the above carboxylic acids such as naphthenic acid, oleic acid, linoleic acid, linolenic acid, soybean oil fatty acids, tall oil fatty acids, etc. Examples thereof include unsaturated aliphatic carboxylic acids, aromatic aliphatic carboxylic acids such as diphenylacetic acid, and aromatic carboxylic acids such as benzoic acid and toluic acid. In the present invention, a zirconium salt of a saturated aliphatic carboxylic acid is preferred. In general, a metal salt of a carboxylic acid can serve as an allophanation catalyst. Examples of the metal include alkali metals such as lithium, sodium, and potassium; alkaline earth metals such as magnesium, calcium, and barium; Examples thereof include transition metals such as manganese, iron, cobalt, nickel, copper, and zinc; boron group metals such as aluminum; carbon group metals such as tin and lead; and titanium group metals such as titanium and zirconium. The present inventors have found that zirconium carboxylate as an allophanate-forming catalyst is less toxic and suppresses side reactions (such as formation of isocyanurate groups leading to higher viscosity and formation of uretdione groups leading to lower durability). It has been found to have performance. Examples of the catalyst poison (e) used in the present invention include inorganic acids such as phosphoric acid and hydrochloric acid, organic acids having a sulfonic acid group, a sulfamic acid group and the like, and known compounds such as esters and acyl halides. No. Next, a specific manufacturing procedure will be described.
The present invention includes the following steps. First step: organic diisocyanate (a) and PPG
The polyol (b) and the alkyl alcohol (c) are charged in an amount such that the isocyanate group becomes excessive with respect to the hydroxyl group, and the urethane is added at 70 to 150 ° C. in the presence of a zirconium carboxylate (d) as an allophanatization catalyst. A step of subjecting the group to an allophanation reaction until substantially no groups are present. Second step: a step of adding a catalyst poison (e) to stop the allophanation reaction. Third step: a step of removing free organic diisocyanate (a) until the content becomes 1% or less. The first step is a step of simultaneously performing a urethanization reaction and an allophanation reaction. Specifically, an organic diisocyanate (a) and a PPG-based polyol (b) and an alkyl alcohol (c) are reacted with an isocyanate. The amount in which the groups are excessive with respect to the hydroxyl groups is
This is a step of simultaneously performing a urethanization reaction and an allophanation reaction at a temperature of about 150 ° C. in the presence of the allophanation catalyst (c). Here, "the amount of isocyanate groups in excess with respect to the hydroxyl groups" means that the isocyanate groups become excessive with respect to the hydroxyl groups when the raw materials are charged. The molar ratio is preferably isocyanate group / hydroxyl group = 8 or more, and particularly preferably 10 to 50. Here, the urethane-forming reaction and the allophanate-forming reaction can be performed separately. However, when a multi-step reaction is performed, the production time becomes longer, and the resulting polyisocyanate may be colored. Therefore, in the present invention,
The one-step reaction, that is, the urethanization reaction and the allophanation reaction were performed simultaneously. In this case, the mass ratio between (b) and (c) is preferably (b) :( c) = 90/10 to 10/90,
Particularly, (b) :( c) = 85/15 to 15/85 is preferable. If the amount of (b) is too large, the solubility of the resulting polyisocyanate in poor solvents tends to decrease. When (c) is too large, the isocyanate content of the resulting polyisocyanate becomes small. The reaction temperature is from 70 to 150 ° C, preferably from 80 to 130 ° C. If the reaction temperature is too low, the reaction time is prolonged, the isocyanurate group in the resulting polyisocyanate increases, and the viscosity tends to be high.
If the reaction temperature is too high, the resulting polyisocyanate tends to be more colored. The reaction time in the first step is usually preferably within 10 hours, particularly preferably within 5 hours. The amount of the allophanate-forming catalyst (d) varies depending on the type thereof, but is preferably 0.0005 to 1% by mass, and more preferably 0.0005 to 1% by mass based on the total amount of the above (a) to (c).
More preferably, it is 1 to 0.1% by mass. The amount of catalyst used is 0.0
When the content is less than 005% by mass, (a) even if the reaction temperature is appropriate, the reaction is slow and requires a long time, the resulting polyisocyanate has a large isocyanurate group content, and tends to have high viscosity. Or (b) In order to obtain an appropriate reaction time, it is necessary to raise the reaction temperature, but allophanate formation does not proceed, and uretdione formation reaction of isocyanate group or isocyanurate formation reaction which is a side reaction progresses. , Easy to have high viscosity. When the amount of the catalyst exceeds 1% by mass, the control of the reaction becomes difficult, and a uretdione-forming reaction or a chemical reaction, which is a side reaction, proceeds, and the viscosity tends to be high. At this time, an organic solvent can be used if necessary. Examples of the organic solvent include aliphatic hydrocarbon-based organic solvents such as n-hexane and octane; alicyclic hydrocarbon-based organic solvents such as cyclohexane and methylcyclohexane; and ketone-based organic solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. solvent,
Ester organic solvents such as methyl acetate, ethyl acetate, butyl acetate, and isobutyl acetate; ethylene glycol ethyl ether acetate; propylene glycol methyl ether acetate; 3-methyl-3-methoxybutyl acetate; ethyl-3-ethoxypropionate; Glycol ether ester organic solvent, diethyl ether,
Ether organic solvents such as tetrahydrofuran and dioxane, halogenated aliphatic hydrocarbon organic solvents such as methyl chloride, methylene chloride, chloroform, carbon tetrachloride, methyl bromide, methylene iodide, dichloroethane, N-methylpyrrolidone, dimethylformamide And polar aprotic solvents such as dimethylacetamide, dimethylsulfoxide and hexamethylphosphonylamide. One or more of the solvents can be used. Even when an organic solvent is used, this organic solvent can be used as a free organic diisocyanate (a) in the third step described below.
Will be removed with it. In the second step, after the allophanate-forming reaction, a catalyst poison (e) is added to stop the allophanate-formation reaction. The timing of adding the catalyst poison (e) is not particularly limited as long as it is after the allophanate-forming reaction. However, when performing thin-film distillation in the method of removing free organic diisocyanate in the third step, after the allophanate reaction and before the distillation. It is preferable to add a catalyst poison (d) to the mixture. This is to prevent a side reaction from occurring due to heat at the time of distillation. The amount of the catalyst poison (e) varies depending on the type of the catalyst and the type of the catalyst, but is preferably 0.5 to 2 equivalents of the catalyst, and particularly preferably 0.8 to 1.5 equivalents. If the amount of the catalyst poison is too small, the storage stability of the obtained polyisocyanate tends to decrease. If the amount is too large, the resulting polyisocyanate may be colored. The third step is a step for removing free organic diisocyanate (a). In the present invention, basically, free organic diisocyanate is present in the product after the allophanation reaction. Since this free organic diisocyanate causes turbidity when it changes in odor or changes over time, the free organic diisocyanate content is 1%.
It is preferable to remove the unreacted organic diisocyanate (a) until the amount becomes less than mass%. The free organic diisocyanate content is a value measured by gas chromatography. As a method for removing free organic diisocyanate, known methods such as distillation, reprecipitation, extraction and the like can be mentioned. A distillation method, particularly a thin film distillation method, is preferable because a solvent and the like do not need to be used. Preferred conditions for thin-film distillation include a pressure of 0.1 kPa or less and a temperature of 10 kPa.
0 to 200 ° C., and particularly preferable conditions are pressure: 0.0
5 kPa or less, temperature: 120 to 180 ° C. The allophanate-modified polyisocyanate thus obtained contains substantially no organic solvent, urethane group, uretdione group, or isocyanurate group. In addition, "substantially" means that it hardly affects the obtained polyisocyanate.
The presence of these traces is within the permissible range of the present invention. The viscosity at 25 ° C. of the allophanate-modified polyisocyanate obtained according to the present invention is 2,000 mP
a · s or less, preferably 100 to 1,800 mP
a · s. The isocyanate content is preferably from 5 to 25% by mass, particularly preferably from 8 to 13% by mass. If the isocyanate content is less than 5% by mass, the crosslink density of the coating film may be low. If it exceeds 25% by mass,
Solubility in poor solvents may decrease. The allophanate-modified polyisocyanate obtained according to the present invention is soluble in poor solvents,
Specifically, polyisocyanate / poor solvent = 50/1 to 1
/ 50 (mass ratio) within the range of no separation or turbidity. In addition, a poor solvent means an aniline point of 10 containing an aliphatic, alicyclic and / or aromatic hydrocarbon-based organic solvent.
70 ° C, preferably 12-65 ° C, more preferably 15 ° C
It is an organic solvent having a weak dissolving power in the range of 60C to 60C. Examples of such organic solvents include methylcyclohexane (aniline point 40 ° C.), ethylcyclohexane (aniline point 44 ° C.), mineral spirit (aniline point 56 ° C.).
C.), turpentine (aniline point: 20 ° C.), etc., and a house generally commercially available as a petroleum hydrocarbon (manufactured by Shell Chemical Co., aniline point: 15 ° C.); Essonafusa No. 6
(Exxon Chemical, aniline point 43 ° C), Loose (Shell Chemical, aniline point 44 ° C), Pegazol 3040
(Manufactured by Mobil Oil, aniline point 55 ° C.). The allophanate-modified polyisocyanate obtained according to the present invention may contain, if necessary, an antioxidant, an ultraviolet absorber, a pigment, a dye, a solvent, a flame retardant, a hydrolysis inhibitor, a lubricant, a plasticizer, Additives such as a filler and a storage stabilizer can be appropriately compounded. The allophanate-modified polyisocyanate obtained according to the present invention is most suitable as a curing agent for two-pack polyurethane coatings. The base agent is a polyol used in the field of two-pack polyurethane coatings, for example, aliphatic hydrocarbon polyols, fluorine polyols, polyether polyols, polyester polyols, polycarbonate polyols, epoxy resins, acrylic polyols, And alkyd polyols and the like, or a mixture thereof. In particular, fluorine polyols and acrylic polyols are more preferable because of their excellent weather resistance. These polyols are produced by known techniques such as solution polymerization and condensation reaction. It is a polyol having two or more hydroxyl groups in one molecule. Even if the main component is of a type that does not dissolve in the poor solvent, it can be used in combination with the polyisocyanate of the present invention, but those that dissolve in the poor solvent can exhibit the performance of the polyisocyanate of the present invention. . The polyol for the base agent is from 1 to 300 mgKOH / g, preferably from 10 to
150mgKOH / g, more preferably 20-100m
Those having a hydroxyl value of gKOH are preferred. Hydroxyl value is 1
When the amount is less than mgKOH / g, the coating film becomes brittle, which is not preferable. When the hydroxyl value exceeds 300 mgKOH / g, the surface smoothness of the coating film may be impaired, or the coating film may be hard and brittle, which is not preferable. The base polyol and polyisocyanate
The equivalent ratio of isocyanate group to hydroxyl group is as follows: isocyanate group / hydroxyl group = 9/1 to 1/9, preferably 8/2 to 2 /
8 If the hydroxyl group is in excess of the above range, the coating film may become brittle. When the amount of the isocyanate group is excessive, the coating film may become brittle because a large amount of the polyisocyanate which does not participate in the crosslinking is present. EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto. In Examples and Comparative Examples, “%” was used.
Means “% by mass”. [Production of Allophanate-Modified Polyisocyanate] Example 1 In a 1 L reactor equipped with a stirrer, a thermometer, a cooler, and a nitrogen gas inlet tube, 810 g of hexamethylene diisocyanate, 155 g of polyol-1 and i 35 g of -propanol and 0.2 g of zirconium 2-ethylhexanoate were charged and reacted at 90 ° C for 3 hours. When the reaction product was analyzed by FT-IR and ¹³ C-NMR, the urethane group had disappeared. 0.1 g of phosphoric acid
A stop reaction was carried out at 50 ° C. for 1 hour. The isocyanate content of the product was 34.3%. This reaction product was subjected to thin-film distillation at 130 ° C. and 0.04 kPa to obtain an allophanate group-containing polyisocyanate P-1. The yield was 42%. The isocyanate content of P-1 is 12.5%, the viscosity at 25 ° C. is 450 mPa · s,
The free HDI content was 0.2%. Also, P-1
Was analyzed by FT-IR and ¹³ C-NMR, the presence of a urethane group was not recognized, the presence of an allophanate group was confirmed, and the uretdione group and the isocyanurate group were traced. The average number of functional groups was 2.3. The average number of functional groups was calculated from the isocyanate content and the number average molecular weight. Examples 2 to 4 and Comparative Examples 1 to 4 Allophanate-modified polyisocyanates P-2 to 8 were obtained using the raw materials shown in Table 1 by the same apparatus and procedure as in Example 1. [Table 1] In Examples 1 to 4, Comparative Examples 1 to 4, and Table 1. [Evaluation of Paint Polyisocyanate] Applied Examples 1-4 and Comparative Examples 1-4 Poor Solvent Dilution Test The obtained polyisocyanate was diluted with Solvent A (poor solvent manufactured by Nisseki Mitsubishi). The tolerance was measured. Table 2 shows the results. [Table 2] Applied Examples 5-8, Applied Comparative Examples 5-7 Volatile oil resistance (compatibility with resin for base resin) Polyisocyanate obtained, Acridic A-82
3 (acrylic polyol solution, solid content = 50%, hydroxyl value = 30 mg KOH / g, manufactured by Dainippon Ink and Chemicals),
Solvent A was mixed with the composition shown in Table 3 and the solid content was 5
A 0% clear paint was prepared. Then, the prepared coating material was applied on a steel plate, and allowed to stand at 20 ° C. and 65% RH for one week to obtain a test piece having a dry film thickness of 30 to 40 μm. The obtained test piece was subjected to solvent A at 25 ° C.
For 4 hours, and then the appearance was confirmed. Table 3 shows the results
Shown in In addition, P-8 is insoluble in the solvent A (less than 0.5 times) in the result of the poor solvent dilution test described above.
Was not evaluated. [Table 3] As can be seen from Tables 2 and 3, the polyisocyanates of the examples had good solubility in poor solvents and good resistance to volatile oils, whereas the polyisocyanates of comparative examples had good solubility in poor solvents.
None of them had good volatile oil resistance. As described above, the polyisocyanate obtained by the present invention was low in viscosity, soluble in poor solvents, and had a sufficient isocyanate content.

Continuation of front page    F term (reference) 4J034 CA02 CB01 CC02 DA01 DB03                       DB04 DB07 DD07 DF01 DF02                       DG03 DG04 DG14 DG15 DG23                       DK02 DP18 HA01 HA07 HB05                       HB07 HB11 HB12 HC03 HC12                       HC13 HC17 HC22 HC46 HC52                       HC61 HC64 HC67 HC70 HC71                       HC73 JA02 JA14 JA32 JA42                       KA01 KA02 KB07 KC02 KC04                       KC07 KC08 KC13 KC16 KC17                       KC18 KC32 KC35 KD02 KD14                       KD21 KD22 KD24 KD25 KE01                       KE02 LB06 QA03 QA05 RA07

Claims (1)

Claims 1. An organic solvent, containing substantially no urethane group, uretdione group, or isocyanurate group, having a viscosity at 25 ° C of 2,000 mPa · s or less and an isocyanate content of 5 to 25 mass%. % Of an allophanate-modified polyisocyanate in the following steps. First step: an organic diisocyanate (a), a polypropylene glycol-based polyether polyol (b) having a nominal number of functional groups of 1 to 3 and a number average molecular weight of 500 to 3,000, and an alkyl alcohol (c) having 1 to 20 carbon atoms. Is added in such an amount that the isocyanate groups become excessive with respect to the hydroxyl groups, and the allophanate-forming reaction is carried out at 70 to 150 ° C. in the presence of the zirconium carboxylate (d) as a catalyst for the allophanate until the urethane group substantially disappears. Process to make it. Second step: a step of adding a catalyst poison (e) to stop the allophanation reaction. Third step: a step of removing free organic diisocyanate (a) until the content becomes 1% or less.