JPH01203356A - Production of polyisocyanate - Google Patents

Abstract

PURPOSE:To obtain the present product excellent in product composition, by redistilling pure MDI separated from a thermal decomposition reaction product of polymethylene polyphenyl polycarbamate under reduced pressure and converting the separated 2,2'- and 2,4'-DMI into a polynuclear substance. CONSTITUTION:A thermal decomposition reaction product of polymethylene polyphenyl polycarbamate is distilled under reduced pressure to separate pure MDI mainly containing 4,4'-MDI (methylene diphenyl diisocyanate). The pure MDI is redistilled under reduced pressure to separate 2,2'- and 2,4'-MDI isomers, which are then reacted with a lower alcohol and converted into the corresponding dicarbamate. The obtained dicarbamate is subsequently reacted with N- phenylcarbamate and a methylenating agent in the presence of an acid catalyst to afford a tri- or polynuclear substance polycarbamate. The resultant polycarbamate is then thermally decomposed and converted into a tri- or polynuclear substance polyisocyanate. Thereby, a polyisocyanate substantially without containing 2,2'-MDI and 2,4'-MDI which have low reactivity and not found useful applications is obtained.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to polymethylene polyphenyl polyisocyanate (hereinafter abbreviated as polyisocyanate) useful as a raw material for producing polyurethane elastomers and coating materials.
Relating to a manufacturing method. More specifically, among the dinuclear methylene diphenyl diisocyanate (MDI), 2.2'-MDI and 2.4'' have not been found to have any useful uses.
- An improved process for the production of polyisocyanates, which makes it possible to obtain polyisocyanate products that are substantially free of MDI and have an excellent product composition.

(Conventional technology) Conventionally, tolylene diisocyanate (TDI) and MDI
Aromatic isocyanate compounds such as these have been industrially produced by a method in which an aromatic nitro compound is reduced with hydrogen to form an aromatic amine, and then phosgene is applied to this to convert the amino group into an isocyanate group. However, this method has problems such as a complicated process, the use of toxic phosgene, and a large amount of hydrogen chloride produced as a by-product. Therefore, many methods for producing aromatic isocyanates without using phosgene have been proposed.

These methods are broadly classified into ① direct method and ② carbamate-mediated method.

The first direct method is a method in which an aromatic nitro compound is reacted with carbon monoxide in the presence of a palladium-based catalyst to obtain an aromatic isocyanate compound. This method has drawbacks such as not only harsh reaction conditions but also low productivity of the catalyst and a tendency for side reactions to occur. Furthermore, although this method can be applied to the production of TD+, it cannot be applied to the production of polynuclear polyisocyanates such as MDI because there are restrictions on the availability of the corresponding nitro compound as a raw material.
Very difficult.

The second method involves reacting an aromatic nitro compound and an alcohol with carbon monoxide in the presence of a platinum group metal catalyst or a selenium catalyst to obtain an intermediate aromatic carbamate, which is then thermally decomposed. This is a method to obtain aromatic incyanate.

The present invention relates to a method for producing polyisocyanate using the second carbamate-mediated method. When producing polyisocyanate by this method, as shown in the reaction formula below, N-phenyl carbamate (+) is condensed and crosslinked with a methylenating agent such as formaldehyde in the presence of an acid catalyst to produce a polynuclear structure polyisocyanate. Methylene polyphenyl polycarbamate (■) (hereinafter simply referred to as polycarbamate) is produced, and this polycarbamate is then thermally decomposed to produce the corresponding polyisocyanate (III).
Manufacture.

(In the formula, m is O or an integer of 1 to 6, R is a carbon number of 1 to 6
lower alkyl groups).

This method differs from the above-mentioned direct method in that it only requires the use of raw materials with a mononuclear structure, and in addition, an excellent method for synthesizing the raw material N-phenyl carbamate from aromatic nitro compounds or aromatic amino compounds has been developed in recent years. , is expected to be an advantageous method for producing polyisocyanates. However, in the conventionally known method, there are problems with the quality of the polyisocyanate product obtained, so this method has not been put into industrial implementation.

(Problems to be Solved by the Invention) In the production of the polyisocyanate having a polynuclear structure produced by the present invention, even if the aim is to produce a dinuclear body MD+ (a compound where m=Q in the above general formula (■)), During crosslinking, it is inevitable that trinuclear or more polyisocyanates (m≧1) are produced as by-products, and among dinuclear polyisocyanates, 4.4”-MDI (4
, 4'-methylene diphenyl diisocyanate), but the isomers 2,2'-MDI (2,2'-methylene diphenyl diisocyanate) and 2.4°-MD
It is inevitable that I (2,4°-methylene diphenyl diisocyanate) will be produced as a by-product. Therefore, the reaction product will be a mixture containing these various polyisocyanates. It is virtually impossible to completely separate each polyisocyanate component by distillation of the reaction product because it is susceptible to deterioration and results in a decrease in product yield and quality.

Therefore, in the production of polyisocyanate, 4,4°-MD I
4. Separate a portion of the product and collect it as a pure MDI product.
The distillation residue containing the remainder of 4'-MDI and 3IA' or higher polyisocyanate is converted into polymeric MDI (or crude MD) without further distillation.
I) It is recovered as a product, and the former is mainly used as a raw material for manufacturing various polyurethane products, and the latter is mainly used as a raw material for manufacturing various polyurethane foams.

As a quality problem with pure MDI, 4.4
In addition to °-MDI, trace amounts of 2,2°-MD, which is an isomer,
I and a considerable amount of 2,4"-MDI may be mixed in. Since 2,2°-MDI and 2,4'-MDI have slightly lower boiling points than 4,4°-MDI, Although it can be separated from 4,4'-MDI by distillation, both of these have low reactivity, so there is currently no significant use for it. Therefore, 2,2"-MDI separated from pure MDI by vacuum distillation. and 2.4゛-MD I is
Currently, it is mixed with polymeric MDI as it is and digested, and since this contamination reduces the reactivity of polymeric MDI, it is commercially desirable to reduce the amount of these isomeric MDI produced as much as possible. has been done.

Against this background, the present invention aims to reduce the production amount of binuclear MDi other than 4,4°-MDI to almost zero in order to improve the quality of polyisocyanate products to a level that is commercially satisfactory. An object of the present invention is to provide a method for producing polyisocyanate.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a method for producing a corresponding polyisocyanate by thermal decomposition of a polycarbamate, in which 2.2'-MDI is produced from a tal thermal decomposition reaction product. and 2
．． step of separating 4'-MDI, (b1 separated 2,2'-MDI and 2,4'-M
reacting DI with a lower alcohol to convert it into the corresponding dicarbamate, (reacting the dicarbamate reaction product obtained in C1 step (b) with N-phenyl carbamate and a methylenating agent in the presence of an acid catalyst; and (d) thermally decomposing the trinuclear or more polycarbamate product obtained in step (C) to convert it into a trinuclear or more polycarbamate.
Provided is a method for producing a polyisocyanate, which includes the step of converting the polyisocyanate into a polyisocyanate having a core or more.

(effect) Next, the method of the present invention will be specifically explained.

In the method of the present invention, the raw material polycarbamate to be subjected to the thermal decomposition reaction is expressed by the above-mentioned formula (■) (where m is 0 or an integer of 1 to 6, and R is a lower alkyl group having 1 to 6 carbon atoms. ).

As mentioned above, this polycarbamate (n) can be prepared from the corresponding N-phenylcarbamate (1) according to a known method.
and a methylenating agent in the presence of an acid catalyst.

As the methylenating agent, formaldehyde or formaldehyde generating substances such as trioxane, paraformaldehyde, and various formals can be used. The above condensation reaction is usually carried out at 50 to 150°C, preferably 60 to 120°C.
C, more preferably from 80 to 100 C, usually in an aqueous medium.

Inorganic acids are generally used as acid catalysts, specifically:
Mineral acids such as sulfuric acid, hydrochloric acid, and phosphoric acid are most commonly used, but acids classified as strong acids such as hydrobromic acid and perchloric acid can also be used. strong organic acids can also be used.

In the method of the present invention, polycarbamate (n) is thermally decomposed to produce the corresponding polyisocyanate (I [ This can be done using the following method. Known thermal decomposition methods include gas phase methods conducted at high temperatures (e.g., Japanese Patent Publication No. 46
-17773 Publication), and the liquid phase method in which it is dissolved in a solvent and poured at a relatively low temperature (e.g., U.S. Patent No. 2,409.712,
Since polycarbamate is a high boiling point substance, it is supplied to the reaction system in the form of vapor in the gas phase method. In addition, since the liquid phase method has a lower reaction temperature and tends to produce fewer byproducts, it is generally preferable to employ the liquid phase method. The reaction conditions can be the same as those for the thermal decomposition reaction in step (d) described below.

- In formulas (1) and (11), specific examples of R include methyl, ethyl, n-propyl, 1so-propyl, n
-butyl, 1so-butyl, tert-butyl, n-pentyl, rhohexyl groups. It is particularly preferred that R is methyl or ethyl.

-a The preferable range of the value of m in formulas (I[) and (III) varies depending on the use of the polyisocyanate, but m-Q, that is, methylene diphenyl dicarbamate (MDU) of a dinuclear substance, has a commercial value. It is generally preferred that the m-0 fraction be as high as possible, as this produces a high MDI.

It is known that the polynuclear composition of the polycarbamate represented by the general formula (II) obtained by the above condensation reaction can be controlled to some extent by the type of methylenating agent and acid catalyst and the reaction conditions, but it is currently not used industrially. Under possible conditions, m
A mixture of a trinuclear body (MDU) of =o and a polynuclear body of mkl or more is generated. Also, all trinuclear bodies are desirable 4
, 4”-MDU isomer (from which the 4.4°
-MDI, that is, pure MDI), and usually about 5 to 20% of the total MDU generation and ffi are 2.4° integral, and 2.2' integral is 0. It exists in a trace amount of about 1%, and the rest is 4.4°-MDU. Polycarbamate is a high boiling point substance, and it is difficult to isolate the trinuclear MDU by distillation or to separate the isomers of MDU industrially, so it contains various polynuclear substances and isomers. The polycarbamate product is fed directly to the pyrolysis step described above.

As a result, polyisocyanates corresponding to the various polynuclear bodies and isomers supplied are produced as thermal decomposition reaction products, and as mentioned above, 2,4"-MD I and 2
．． A problem arises in that 2'-MDI isomers coexist in pure MDI.

According to the method of the present invention, such thermal decomposition reaction products are treated through the steps detailed below to remove polynuclear 2,4'-MDI and 2,2'-MDI isomers coexisting in pure MDI. By converting these isomers into polymeric MDI, we aim to effectively utilize these isomers and improve the quality of pure MDI and polymeric MDI.

Separation step of 2.2°-MDI and 2.4'-MDI from the tal pyrolysis reaction product: The pyrolysis reaction product is first treated by conventional means such as distillation in the same way as in the conventional separation of pure MDI. Then, MDI is distilled off and separated from the polynuclear substance having a higher boiling point. The distillation residue is recovered as polymeric MDI. Similar to traditional pure MDI recovery, this distillation is a
It is not necessary to completely separate the MDI, and a portion of the MDI may remain in the polymeric MDI of the distillation residue.

Additionally, the MDI fraction of the distillate is redistilled under reduced pressure to extract the lower boiling point 2.2'-MDI and 2.4
°-MDI is distilled out, substantially 4.4'-MDI
A distillation residue consisting of This 4.4'-M DI
If necessary, the MDI is further purified (e.g., distilled, recrystallized, etc.) and recovered as pure MDI.

In these distillations, the temperature is kept as low as possible, for example, 230°C or lower, preferably 1.5°C or lower, in order to prevent a reduction in yield or purity due to side reactions of polyisocyanates at high temperatures.
It is important to complete the operation at a temperature below 50° C. and in as short a time as possible. Note that the thermal decomposition solvent is recovered by vacuum distillation prior to this separation.

(bl2, 2°-MDI and 2.4'-MDI
to the corresponding carbamate: Step (2,2'-MDI and 2.4 separated with al
'-MDI was esterified by reaction with a lower alcohol, and the corresponding 2,2'-MDU and 2,4''
- Convert to MDU.

It is known that aromatic isocyanates easily react with alcohols, especially primary and secondary alcohols, to form carbamates, that is, aromatic carbamate esters. This reaction proceeds simply by mixing the two, and does not require any particular catalyst.

The alcohol used is one represented by the general formula ROH (wherein R represents a lower alkyl group having 1 to 6 carbon atoms), and R is usually a fastener represented by the above-mentioned general formula (II). This is common to R5 inside.

Specific examples of R include methyl, ethyl, n-propyl, 1so
-propyl, n-butyl, 1so-butyl, 5eC-butyl, tert-butyl, n-pentyl, n-hexyl and the like. Among these, alcohols in which R is methyl or ethyl, i.e., methanol or ethanol, are particularly preferred; 2 obtained by this esterification reaction;
2'-methylene diphenyl dicarbamate (MDU)
And 2,4'-MDU is separated from unreacted alcohol and solvent by distillation or the like, and is used as a reaction raw material for the next polynuclear formation.

(c12.2'-MDU and 2.4°-MDU
Polynuclearization step: Step (2,2”-MDU obtained in bl and 2,4°
-MDU is reacted with N-phenyl carbamate and a methylenating agent in the presence of an acid catalyst to convert it into a trinuclear or more polycarbamate.

When a dinuclear MDU is treated with a methylenating agent (such as formaldehyde) in the presence of an acid catalyst, a tetranuclear body is produced by condensation cross-linking of two molecules of MDU with the methylenating agent. - When phenyl carbamate coexists, trinuclear dimethylene triphenyl tricarbamate is produced by condensation of MDU and N-phenyl carbamate with a methylenating agent. In addition, methylene diphenyl dicarbamate (MDU) is also produced as a result of condensation cross-linking between N-phenyl carbamates, so a mixture containing these various components is obtained in this polynuclear formation step. The composition of the product depends on the charging ratio of raw materials, reaction conditions, etc.

This step is based on the above-mentioned general formula (1) in the main process.
can be carried out in the same manner as the condensation reaction from N-phenyl carbamate to polycarbamate of general formula (n),
The reaction conditions such as the methylenating agent, acid catalyst, reaction temperature, and solvent used may also be the same.

That is, as the methylenating agent, formaldehyde or any compound that generates formaldehyde during the reaction (paraformaldehyde, trioxane, formals, etc.) can be used.6 Usually, an aqueous formaldehyde solution is used mainly for economic reasons.

As the acid catalyst, as mentioned above, an inorganic acid such as a mineral acid or a strong acid is generally used. Usually, hydrochloric acid or sulfuric acid is particularly preferably used.

The reaction temperature is usually 60 to 120°C, particularly preferably 8°C.
The reaction temperature is in the range of 0 to 100°C, and the reaction time depends on the reaction conditions, but is usually 0.1 to 10 hours, preferably 0.5 hours.
~5 hours.

When carrying out this polynuclear formation process of MDLI by one condensation reaction in a 0-batch method, which can be carried out in a batch method, a continuous method, or a semi-continuous method, for example, a predetermined amount of MDU and N-phenylcarbamate are placed in a reactor equipped with a stirring device. , inorganic acid and an appropriate amount of water if necessary, and completely remove nitrogen while stirring.
- Melt the phenyl carbamate and bring it into suspension.

After gradually adding an aqueous formaldehyde solution to this solution while stirring, the temperature is raised to a predetermined temperature and maintained at that temperature for the time required to complete the reaction.

After the reaction is complete, the oil and water may be separated directly, but it is also possible to separate the oil and water after cooling the system to room temperature and adding an appropriate organic solvent to completely dissolve the product. It is valid. If necessary, the obtained product is further subjected to impurity removal treatment using an ion exchange resin or the like, and then subjected to the next thermal decomposition reaction.

At this time, the N-phenyl carbamate remaining in the condensation product may be separated and recovered by distillation, but it is also possible to use part or all of it as a raw material for the thermal decomposition reaction while coexisting in the product. be.

Note that this step of converting MDU into a polynuclear body by condensation is carried out in a separately provided reactor by allowing a methylenating agent and N-phenyl carbamate to act on the MDU obtained in the step (bl) in the presence of an acid catalyst. However, as an alternative method, the condensation reaction of the main process described above [i.e. - N in formula (1)]
- condensation reaction of phenyl carbamates to polycarbamates of general formula (n)) can also be carried out in a reactor. In the latter case, the MDU obtained in step) is charged to the condensation reactor of the main process together with N-phenyl carbamate as part of the feed and simultaneously with the condensation reaction of N-phenyl carbamate to polycarbamate. Perform multinuclearization of MDU.

(d) Pyrolysis step of polycarbamate having 3 or more nuclei: Step (The product containing the polycarbamate having 3 or more nuclei obtained in step C1 is thermally decomposed to produce the corresponding polyisocyanate having 3 or more nuclei. Converted to containing products.

This step, as described above, can be carried out under the same reaction conditions as the thermal decomposition step of the main process, ie, the thermal decomposition step of the polycarbamate of the formula (II) to the polyisocyanate of the general formula (II). In addition, when the above step (e) is carried out in the condensation reactor of the main process, the step (d
The pyrolysis of 1 is naturally also carried out in the pyrolysis reactor of the main process. On the other hand, when the condensation reaction of step 10) is carried out in a reactor different from the condensation reactor of the main process, step (d
The pyrolysis of the polynuclear polycarbamate of 1 can be carried out in a reactor separate from the reaction system of the main process, or it can be charged as part of the feedstock to the pyrolysis reactor of the main process. It can also be implemented by

The thermal decomposition reaction is preferably carried out in a liquid phase by dissolving the raw material polycarbamate in a solvent, in which case,
The polycarbamate product containing trinuclear or higher polycarbamate obtained in step (C1) is thermally decomposed into the corresponding polyisocyanate and alcohol in a temperature range of 180 to 300°C, preferably 230 to 260°C. Convert, 3
A polymeric polyisocyanate product containing more than the core polyisocyanate is obtained. If the thermal decomposition temperature is less than 180°C, it is difficult to obtain a sufficient reaction rate. On the other hand, if the thermal decomposition temperature exceeds 300°C, various side reactions occur and are undesirable. The reaction can be carried out under any conditions, and any of the batch, continuous, and semi-continuous methods can be adopted as the reaction method.

The solvent used in the thermal decomposition reaction is liquid at the reaction temperature and pressure, can completely dissolve the raw material polycarbamate and the product polyisocyanate, is substantially inert to the polycarbamate and polyisocyanate, and is compatible with the reaction conditions. It is thermally and chemically stable at low temperatures.

Furthermore, a solvent having a boiling point in the range of 100 to 350°C, preferably 120 to 300°C at normal pressure is used.

If the boiling point is 350° C. or higher, the distillation temperature when removing the solvent by distillation from the polyisocyanate solution in the next solvent removal step will be high, and side reactions of the polyisocyanate may occur.

If the boiling point is below 100°C, the efficiency of separation from alcohol produced during thermal decomposition will be poor.

Specific examples of compounds that can be used as solvents in the thermal decomposition process include higher alkanes such as didecane, dodecane, and tetradecane; butylbenzene, cyclohexylbenzene,
Substituted benzenes and substituted naphthalenes such as dodecylbenzene and methylnaphthalene isomers; alicyclic aromatic hydrocarbons such as tetralin and decalin; halogenated aromatic hydrocarbons such as dichlorobenzene isomers and chloronaphthalene isomers; Aromatic ethers such as diphenyl ether, dibenzyl ether, diphenyl ether, butylphenyl ether, benzyl ethyl ether, 1,2'-dibutoxyethane, diethylene glycol diethyl ether;
Aromatic nitro compounds such as nitrobenzene and nitrotoluene isomers; ketones such as acetonyl acetone, isophorone, and acetophenone; esters such as γ-butyrolactone; carbamates that do not react with isocyanates such as ethylene carbonate and propylene carbonate; mixture, among which cyclohexylbenzene, dodecylbenzene, O-dichlorobenzene, 1-
Chloronaphthalene and the like are preferred.

The preferable concentration range of the raw material polycarbamate in the above solvent depends on the reaction method, but is in the range of 1 to 80% by weight, preferably 3 to 50% by weight. If the concentration is less than 1% by weight, equipment costs and utility costs will be large and it is uneconomical, and if the concentration exceeds 80% by weight, side reactions will occur significantly and the yield of the target polyisocyanate will be greatly impaired.

A catalyst may be used for this thermal decomposition, but the reaction proceeds satisfactorily even without the presence of a catalyst.However, the thermal decomposition reaction from polycarbamate to polyisocyanate is a reversible reaction, so It is advantageous for the reaction to proceed by removing the alcohol produced outside the system.An inert gas may be introduced into the reaction system as a carrier gas to promote the removal of alcohol outside the system, or a polycarbamate with a low boiling point may be introduced into the reaction system. It is also possible to add an organic solvent inert to the polyisocyanate to the reaction system and use the generated vapor as a carrier gas.

A mixture consisting of the alcohol, reaction solvent, and part of the polyisocyanate product evaporated from the reactor is entrained in solvent vapor or a carrier and led out of the system as a vapor phase, and then subjected to a fractional distillation or fractional condensation operation in a distillation column. , alcohol + solvent and polyisocyanate + solvent, the latter being recycled to the reaction system. Alternatively, a condenser maintained at a suitable temperature can be placed at the top of the reactor to condense the polyisocyanate and most of the solvent back into the reactor.

A polyisocyanate product containing trinuclear or more polyisocyanate produced by a thermal decomposition reaction is obtained by separating it from the thermal decomposition reaction mixture by a conventional method.For example, the solvent is distilled off under reduced pressure, and the remaining polyisocyanate is a distillate containing MDI as a main component by distilling it under a higher degree of reduced pressure,
It is separated into a fraction whose main component is polyisocyanate having three or more nuclear bodies. The latter fraction is directly recovered as polymeric MDI, and the former fraction, if present, contains 2°2°
-MDI and 2.4'-MDI are distilled off under reduced pressure to produce high-purity 4.4"-MDI, that is, pure MD
Collect I.

The alcohol distilled from the pyrolysis reactor can be recycled in the esterification step of step lbl.

If processes [C1 and (dl) are carried out in the reaction step of the main process, a large amount of alcohol will be recovered, so some of it will be reused in the process, and the rest will be recycled to N-phenyl, which is the raw material for producing polycarbamate. The recovered solvent can be reused in the thermal decomposition reaction.

(Example) Next, an example will be described. Various components were analyzed as follows. The conversion rate, yield, and polynuclear distribution of N-phenyl carbamate, polycarbamate, and polyisocyanate were determined by taking a portion of the reaction mixture, diluting it with dry methanol and/or dry tetrahydrofuran, and then performing high-performance liquid chromatography and It was determined based on the analysis results by gel permini-silane chromatography. In the following examples, yields are based on weight unless otherwise noted.

(A) Polycarbamate Synthesis 300 equipped with stirrer, cooling tube, thermometer and dropping funnel
In a vm flask, add 100 g of water and 148 g of 96% sulfuric acid.
2 ethyl-N phenyl carbamate (EP C) 10
0 g was charged and heated to 50°C, 14.7 g of a 37% formaldehyde aqueous solution was added over 10 minutes, the mixture was heated to 90°C, and the line combination reaction was carried out at 90°C for 2 hours.0 Reaction completed. After that, 100 g of toluene was added to completely dissolve the contents. After thoroughly washing the toluene layer with pure water, the toluene was removed using a rotary evaporator, and the resulting solid was further purified by RPC under reduced pressure. I left. The residue was dissolved in methanol, and a glass column (diameter 30 x height 1000 g) filled with a cation exchange resin (Dowex'50W-X8500 g) and an anion exchange resin (Dowex'1-X8500 g) was prepared.
(Tsuru) to remove impurities. Methanol was distilled off from the effluent to obtain 63 g of solid polycarbame as a condensation product. The composition of this is 4.4'
-MDUlo, 73.1%, 2.4° -MDUlo,
3%, trinuclear or more polycarbamate 12.5%, and other 4.1%.

CB) Pyrolysis of polycarbamate Equipped with a stirrer, a thermometer, a pressure gauge, a packed distillation column (the top of which is connected to a cooling pipe, a pressure holding valve, and a receiver), a gas blowing pipe with a preheater, and a solvent supply pipe. Volume 500 m
In a stainless steel pressure-resistant reactor, 0-dichlorobenzene 2
70 g of the polycarbamate product obtained above were charged, and nitrogen gas was pressurized to 2.9 kg/cIa-G.

It was heated under a constant pressure of 2.9 kg/- while passing nitrogen gas preheated to 100° C. at a flow rate of 3Q+nQ/win. Since the reaction system boiled at 246°C, the temperature of the reaction system was increased by 0.1°C/min by gradually increasing the system pressure from this point on. Since the solvent O-dichlorobenzene was distilled out along with the ethanol by-produced in the reaction, O-dichlorobenzene preheated to 150°C was replenished to adjust the liquid level to be constant during the reaction period.

After 1.5 hours (reaction system temperature 255°C), heating was stopped,
After cooling, the contents were analyzed. The composition of the reaction product excluding the solvent is 4.4'-MD I 70.7%, 2.4
'-MDI 10.0%, trinuclear or higher polyisocyanate 12.2%, others 6.0%, among others 3.1% 4.4"-methylene diphenyl monoisocyanate mono Carbamate (MMI) was included. Unreacted polycarbamate was not detected. Dinuclear 4.4"-MDI and 4.4"-MM
The yields based on 4.4'-MDU in the raw material of I were 94.5 mol% and 1.2 mol%, respectively.

Separation of Isocyanate 2.4'-MDI [Step (a)]
The thermal decomposition product obtained in Example 1 was distilled under reduced pressure to completely remove the solvent, 0-dichlorobenzene, to obtain 126 g of a brown viscous liquid as a residue. This product was distilled using a rotary vane type thin film distiller at 90°C and 11 g in 1 mm, and 3.9 g of crude MDI was distilled and recovered. This crude MDI contained 0.5 g (13%) of 2,4°-MDI.
However, 2.2'-MDI was not observed. Next, 2111 g of this crude MDI was distilled under reduced pressure at 210°C to obtain 0.5 g of 2,4°-MDI (
83%) and 4.4'-MD I 0.1g (17
%) was collected.

(B) Esterification of 2.4'-M D I [Step (
b)] 0.6 g of the distillate of the above MDI mixture was mixed with 10 g of ethanol, and the mixture was reacted at 80° C. for 1 hour with stirring. The resulting reaction solution was concentrated to dryness using a rotary evaporator to obtain 7 g of 2,4°-MDUo.
, a white solid consisting of 0.1 g of 4,4''-MDU0
．． 8g was obtained.

(C) Multinuclearization of MDU [Step (C)] 0.5 g of the MDU mixture obtained in CB) and 5 g of RPCo were added to a flask (100 cm) equipped with a thermometer, a condenser, and a dropping funnel.
3), further added 50 g of 65% sulfuric acid, and heated while stirring with a magnetic stirrer. When the temperature reached 50°C, 0.18g of 37% formaldehyde aqueous solution was added from the dropping funnel, heated to 100'C, and reacted for 2 hours.

After the reaction was completed, 50 cIaff of toluene was added to dissolve the reaction product, the sulfuric acid layer was separated, and the organic layer was washed with hot water.
A residue of 1°Og was obtained by concentration to dryness. As a result of analysis, RP
The C conversion rate was 97.0%. The composition of the product is RP
C was 1.5%, dinuclear bodies were 60.5%, and polynuclear bodies of trinuclear bodies or more and intermediates were 38.0%. The proportion of dinuclear bodies is 94.8% for 4.4'-MD U and 2.4'-MD
U was 5°0%.

(D) Pyrolysis of polycarbamate [Step (d)] (C)
Distillation under reduced pressure from the condensation product obtained in the same manner as
Thermal decomposition reaction was carried out in the same manner as in Example 1 (B), except that 11 g of EPC-free polycarbamate product was charged at 11 g of °C/1 mm. The composition of the reaction product excluding the solvent is 4,4°-MDI 55%, 2.4°-MDI 3.0
%, trinuclear or more polyisocyanates and intermediates are 4
It was 2%. That is, the proportion of 2,4°-MDI, which accounted for 83% of the starting material used in step (B), could be significantly reduced.

(Effects of the Invention) According to the present invention, the isomers with low reactivity that are inevitably produced together with 4,4'-MDI which is the target substance in the production of MDI, namely 2.4'-MDI and 2.2'-MDI can be efficiently converted into useful trinuclear or higher polyisocyanates by utilizing the reversible reaction of isocyanate + alcohol≠carbamate, which is a feature of the carbamate-mediated method. As a result, the amount of less reactive MDI isomers that have been mixed and digested in so-called polymeric MDI is significantly reduced compared to conventional methods, and the quality of polymeric MDI is improved. Thus, the present invention makes it possible to economically produce polyisocyanates of excellent quality.

Claims (1)

[Claims] (1) In a method for producing a corresponding polymethylene polyphenyl polyisocyanate by thermally decomposing a polymethylene polyphenyl polycarbamate, (a) 2,2'-methylene diphenyl diisocyanate and 2,4' are produced from the thermal decomposition reaction product. - separating methylene diphenyl diisocyanate; (b) reacting the separated 2,2'-methylene diphenyl diisocyanate and 2,4'-methylene diphenyl diisocyanate with a lower alcohol to convert them into the corresponding dicarbamates; c) A step of reacting the dicarbamate reaction product obtained in step (b) with N-phenyl carbamate and a methylenating agent in the presence of an acid catalyst to convert it into trinuclear or more polymethylene polyphenyl polycarbamate. , and (d) a step of thermally decomposing the trinuclear or more polycarbamate product obtained in step (c) to convert it into trinuclear or more polymethylene polyphenyl polyisocyanate. A method for producing polymethylene polyphenyl polyisocyanate.