JPH01121259A - Production of polyisocyanate - Google Patents

Abstract

PURPOSE:To obtain a compound useful as a raw material for producing polyurethane economically and advantageously, by subjecting polymethylene polyphenyl polycarbamate to thermal decomposition reaction in a liquid phase at two stages. CONSTITUTION:Polymethylene polyphenyl polycarbamate (hereinafter abbreviated as polycarbamate) is heated in the absence of a catalyst in an organic solvent such as chloronaphthalene under normal pressure at 150-350 deg.C and decomposed into polymethylene polyphenyl polyisocyanate (abbreviated as polyisocyanate) and an alcohol. The prepared reaction product is distilled and separated into a low-boiling component comprising methylene diphenyl diisocyanate of substantially binuclear compound and a high-boiling component containing a polynuclear polyisocyanate of tri- or more nuclear compound, an unreacted polycarbamate raw material and a carbamate isocyanate compound of intermediate. The high- boiling component discharged from the bottom of the column is further thermally decomposed by another reactor to give the aimed substance.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing polymethylene polyphenyl polyisocyanate. The present invention relates to a method for producing isocyanate.

Polymethylene polyphenyl polyisocyanate (hereinafter referred to as
Polyisocyanates (abbreviated simply as polyisocyanates), especially the dinuclear methylene diphenyl diisocyanate (MDI), are useful materials as raw materials for the production of polyurethane elastomers and coating materials, and the amounts used in this application are highly volatile and toxic. It currently surpasses tolylene diisocyanate (TDI), which has been a problem due to its strong oxidation, and is being mass-produced on an industrial scale.

(Prior Art) Conventionally, aromatic isocyanates have generally been produced by reacting phosgene with an aromatic amine obtained by hydrogen reduction of an aromatic nitro compound. 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, methods for producing aromatic isocyanates that do not use phosgene have been studied for the past 20 years.

These methods are roughly divided into two types: (l) direct method and (2) carbamate-mediated method.

The direct method is a method in which an aromatic nitro compound is reacted with carbon monoxide in an inert solvent mainly in the presence of a palladium catalyst to directly obtain an aromatic isocyanate. 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, it is difficult to directly apply this method to the production of the above polyisocyanate.

The second carbamate synthesis method involves reacting an alcohol and an aromatic nitro compound with carbon monoxide in the presence of a platinum group metal catalyst or a selenium catalyst to form an aromatic carbamate, and then thermally decomposing the carbamate. This is a method for obtaining the corresponding aromatic isocyanate.

The present invention relates to a method for producing polyisocyanate using such a carbamate-mediated method.

According to this method, as shown in the reaction formula below, N-phenyl carbamate (I) is crosslinked by condensation with a methylenating agent such as formaldehyde to form polymethylene polyphenyl polycarbamate (hereinafter simply referred to as polycarbamate). ) (■) and then pyrolysis of this polycarbamate yields the corresponding polyisocyanate (I[[
) is obtained.

(In the formula, m is 0 or an integer of 1 to 6, R is a carbon number of 1 to 6
lower alkyl group).

This method is expected to be an advantageous method for producing polycarbamates or polyisocyanates because an excellent method for synthesizing N-phenyl carbamate as a raw material from a nitro compound or an amino compound has been developed in recent years. However, when the second step of this process, the pyrolysis step of the polycarbamate, is carried out by conventionally known pyrolysis methods, the reaction rate and the yield and quality of the polyisocyanate product are unsatisfactory. has not yet reached industrial implementation.

Known methods for thermal decomposition of carbamate II are broadly divided into methods performed in a high-temperature gas phase and methods performed in a relatively low-temperature liquid phase.

The former gas phase method is described, for example, in Japanese Patent Publication No. 46-17773, but it requires a short reaction time under conditions of high temperature and high vacuum, and cannot be applied to polycarbamates with high boiling points. Practically impossible.

The liquid phase pyrolysis method is described, for example, in JP-A Nos. 51-19721, 52-19624, 54-39002, and 57-
As explained in Japanese Patent No. 158747, thermal decomposition is carried out in the presence of a catalyst and/or under reduced pressure or vacuum conditions. As a result, there are problems such as post-processing such as separation and recovery of the catalyst being required, which complicates operations, and reduced pressure or vacuum resulting in significant operational constraints, as well as oversized and complicated equipment. .

In the production of polyisocyanates, high-boiling polyisocyanates (polynuclear bodies of trinuclear bodies or more, m≧ in the general formula (III) above) that cannot be substantially purified by distillation are used.
A reaction product containing 1) is produced. From this reaction product, the binuclear MDr (m=0 in the above formula) is usually separated by distillation to obtain a pure MDI product, and the remainder is recovered as a crude MDI product, both of which are used as raw materials for polyurethane production. . Crude MDI is MD that has not been distilled.
In addition to this, it contains polynuclear substances with higher boiling points.

In this way, when distillation bottom liquor is also used as a product, unreacted polycarbamate raw materials and residual isocyanate carbamate intermediates in which only a portion of the carbamate groups have been converted to isocyanate groups (these are high-boiling substances) Therefore, it is necessary to avoid as much as possible the residual MDI (which would inevitably be included in crude MDI), and it is essential to complete the above thermal decomposition.

To achieve this, it is conceivable to carry out heating at high temperatures for a long time until the thermal decomposition is completed, but this may cause undesirable concurrent or sequential side reactions, producing many types of resin-like high-boiling substances. This results in a decrease in the yield and quality of the polyisocyanate product, resulting in a product unsuitable as a raw material for polyurethane.

(Problem to be solved by the invention) Therefore, without using a catalyst or reduced pressure conditions, the thermal decomposition reaction of polycarbamate can be completed in as low a temperature as possible in a short time, and the remaining amount of unreacted raw materials and intermediates can be substantially reduced. It is extremely important to achieve zero in the establishment of polyisocyanate production technology using the carbamate-mediated method. The present invention
The purpose is to solve this technical problem.

In this regard, for the production of TDI via a carbamate method, the bottoms containing the unreacted tolylene carbamate raw material and intermediate tolylene carbamate isocyanate after distilling off the target TDI are optionally used at a high boiling point. It has been proposed (Japanese Patent Publication No. 46-17773) to circulate it into a pyrolysis reactor together with an inert solvent.
At this time, a method of removing heavy by-products using a stripping method is also known (Japanese Patent Laid-Open No. 51-29445). Furthermore, in order to increase the reaction rate and effectively utilize the catalyst, a method has been proposed in which a liquid containing an intermediate and a catalyst is circulated (Japanese Patent Laid-Open No. 19624/1983).

However, in the production of the polyisocyanate based on MDI of the present invention, as mentioned above, distillation bottoms are also recovered and commercialized as crude MDI, and pyrolysis raw materials and intermediate carbamate group-containing compounds are also used. Since it is a high-boiling point substance, it is difficult to further purify distillation bottoms by distillation unless an extremely high vacuum is used.
It is inappropriate to remove unreacted raw materials and intermediates by applying the above-mentioned conventional techniques related to TDI, and the polyisocyanate thus obtained becomes a raw material for polyurethane production as a result of long-term exposure to high temperatures. It is not possible to meet the required quality.

(Means for Solving the Problems) As a result of intensive studies to achieve the above object, the present inventors have distilled and separated pure MDI consisting essentially of dinuclear polyisocyanate from the thermal decomposition reaction mixture. Later, it was discovered that by thermally decomposing the bottoms again in a separate thermal decomposition reactor, unreacted polycarbamate and intermediates could be substantially completely converted into polyisocyanate in a short period of thermal decomposition. , completed the present invention.

Here, the gist of the present invention is to provide a method for producing polyisocyanate by liquid-phase pyrolysis of polycarbamate, in which the obtained pyrolysis reaction product is distilled, and the reaction product is converted into substantially dinuclear methylene diphenyl. It is separated into a low-boiling point component consisting of diisocyanate and a high-boiling point component containing not only binuclear polyisocyanate but also polynuclear polyisocyanate of trinuclear or more, unreacted polycarbamate raw material, and intermediate carbamate isocyanate compound, and is separated from the bottom of the column. This method of producing polyisocyanate is characterized in that the discharged high-boiling components are further subjected to liquid phase pyrolysis in a reactor provided separately from the pyrolysis reactor for raw material polycarbamate.

(effect) The present invention will be explained in detail below.

In the method of the present invention, the raw material polycarbamate to be subjected to the thermal decomposition reaction is the -killing (■) (in the formula, 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 (■) was prepared by converting it into the corresponding N-phenylcarbamate (1) according to a known method.
and a methylenating agent (formaldehyde or a substance that generates formaldehyde) in the presence of an acid catalyst.

In general formulas (I) and (11[), specific examples of R include methyl, ethyl, n-propyl, 1so-propyl,
n-butyl, 1so-butyl, 5ec-butyl, ter
These are t-butyl, n-pentyl, and n-hexyl. Preferably R is methyl or ethyl. In addition, the preferable range of the value of m in -kill reduction (I[) varies depending on the use of the polyisocyanate, but MDI with high commercial value is produced from methylene diphenyl cicabamate (MDU) where m = Q. , 8 in which the proportion of m=Q is as high as possible is generally preferred.

As the methylenating agent, formaldehyde or a substance that generates formaldehyde is used. A substance that generates formaldehyde is a substance that generates formaldehyde by decomposition etc. under the above condensation reaction conditions,
Examples include trioxane, paraformaldehyde, methylal and other formals.

Usually, an aqueous formaldehyde solution such as formalin is used, mainly for economic reasons.

The condensation reaction is usually carried out at a temperature of 50 to 150°C.

As the acid catalyst, mineral acids such as sulfuric acid and hydrochloric acid, Lewis acids such as boron trifluoride, and organic acids such as methanesulfonic acid can be used, and strong acids are preferably used. If desired, a polar organic solvent may be present.

The polycarbamate reaction product produced by condensation can be obtained as a solid by removing the catalyst, unreacted raw materials, water, and other reaction solvents from the condensation reaction mixture.

It is known that the polynuclear composition of the polycarbamate represented by the general formula (II) obtained in this way can be controlled to some extent by the types of methylenating agent and acid catalyst and the reaction conditions, but in any case, m- A mixture of trinuclear bodies with m≧1 and polynuclear bodies with m≧1 is generated. As mentioned above, polycarbamate is a high boiling point substance and it is difficult to isolate the trinuclear substance by distillation, so it is usually used as a raw material for the thermal decomposition reaction of the present invention in the form of a mixture of trinuclear bodies and polynuclear bodies. do. Further, unreacted N-phenyl carbamate in the reaction product is usually removed by distillation, but it can also be subjected to the thermal decomposition reaction while remaining in whole or in part.

According to the method of the invention, the polycarbamates thus prepared (old) are pyrolyzed in the liquid phase and the corresponding general formula (Iff
) to produce a polyisocyanate represented by (wherein,
m and R have the same meanings as above).

This reaction is carried out by heating the polycarbamate to high temperature in an organic solvent, preferably in the absence of a catalyst, to thermally decompose it into polyisocyanate and alcohol. The generated alcohol can be collected and suitably reused for producing N-phenyl carbamate, which is a raw material for the condensation reaction.

The thermal decomposition reaction temperature is generally 150 to 350°C, preferably 200 to 300°C, particularly preferably 220 to 28°C.
It is within the range of 0°C. The reaction time depends on the type of raw material polycarbame, reaction method, reaction conditions, especially reaction temperature.
Generally it is within the range of 5 minutes to 10 hours.

The concentration of raw polycarbamate in the reaction solvent ranges from 1 to 8% by weight, more preferably from 2 to 50% by weight.

The above thermal decomposition reaction is carried out batchwise or continuously under reduced pressure, normal pressure or increased pressure. A continuous type reaction, particularly a flowing type reaction, is preferable because it can obtain desired reaction results in a shorter time than a batch type reaction. Further, reaction at normal pressure is advantageous since it requires less equipment.

A sufficient reaction rate can be obtained even without the presence of a catalyst. Furthermore, in the absence of catalysts, the work-up of the reactants can be significantly simplified.

The solvent used for the pyrolysis of the present invention is liquid at the reaction temperature and pressure, capable of completely dissolving both the raw polycarbamate and the polyisocyanate product, and thermally and chemically stable under the reaction conditions. It is something. In this sense, organic solvents having a normal pressure boiling point of 150 to 330°C, particularly 170 to 270°C are suitable.

Solvents with a boiling point exceeding 330°C can also be used, but when recovering the solvent from the reaction mixture by distillation after the reaction, the boiling point will be close to that of the polyisocyanate produced, making it difficult to separate the solvent, and the distillation temperature will be high. This is not recommended because it tends to cause quality deterioration of the polyisocyanate product due to side reactions. On the other hand, the boiling point is 15
When a solvent at 0° C. or lower is used, pressurization is required to maintain the reaction system in a liquid phase, which complicates the equipment and operation.

Specific examples of solvents that can be used in the thermal decomposition reaction of the present invention are listed below: Higher alkanes such as decane, n-hexadecane, and n-octadecane; O-xylene, m-xylene, p-
Unsubstituted or substituted aromatic hydrocarbons such as xylene, ethylbenzene, cumene, diisopropylbenzene, cyclohexylbenzene, naphthalene, nitrohenzene, 100 benzene, methylnaphthalene, chloronaphthalene, diphenyl and substituted diphenyl, diphenylmethane, anthracene, phenanthrene; methylcyclohexane, 5- to 7-membered alicyclic hydrocarbons such as cyclopentane. In addition, ethers and polyethers (e.g. diphenyl ether) that have no reactivity with isocyanates (
(including cyclic ethers), ketones (eg, methyl isobutyl ketone), esters (eg, dioctyl phthalate), and sulfur polyesters of these compounds can also be used as solvents.

Since the thermal decomposition reaction of polycarbamate is a reversible reaction,
In order to shift the chemical equilibrium in the direction of decomposition and prevent recombination of the produced polyisocyanate and alcohol, it is preferable to continuously remove at least one component of the products from the reaction system. To facilitate this removal,
An inert gas carrier can be introduced into the reaction system. Alternatively, low-boiling organic solvents may be used as carriers, which act in a similar manner. Examples of the inert gas include nitrogen, helium, argon, carbon dioxide, methane, ethane, propane, etc., and these can be used alone or in combination. Examples of low boiling point organic solvents are halogenated hydrocarbons such as dichloromethane, chloroform, carbon tetrachloride; lower hydrocarbons such as pentane, hexane, heptane, benzene; ethers such as tetrahydrofuran, dioxane, etc., which are used as reaction solvents. It can be mixed in advance or used while being fed separately and continuously during the reaction.

In the thermal decomposition reaction of the present invention, the most suitable reaction method is adopted depending on the relationship between the boiling points of the alcohol and polyisocyanate to be produced, the reaction solvent used, and the carrier organic solvent.

For example, if the reaction solvent has a boiling point between that of the polyisocyanate produced and the alcohol, and an inert gas is used as a carrier, the alcohol produced in the solution, the reaction solvent, and some polyisocyanate are heated in an inert gas stream. A reaction in which most of the polyisocyanate and reaction solvent are condensed and returned to the reactor using a condenser installed at the top of the reactor, and the alcohol and some of the reaction solvent are taken out of the system. There are several possible methods. When there is a sufficient difference in boiling point between the reaction solvent and the polyisocyanate product, distillation of the produced polyisocyanate can be suppressed and the entire amount can be retained within the reaction system.

The treatment of the resulting pyrolysis reaction mixture is carried out according to the invention as follows. Preferably, after removing the solvent, the reaction mixture is distilled under conditions such that low-boiling components consisting essentially of binuclear MDI are distilled out.

From the bottom of the distillation column, high-boiling components containing trinuclear or higher polynuclear polyisocyanate, raw material polycarbamate, reaction intermediate carbamate isocyanate compound, and residual MDI are recovered as bottoms. The distilled low-boiling components are further purified if necessary to obtain 4.4'
-Recover as pure MDI consisting of MDI and use it as a raw material for polyurethane.

The distillation column bottoms have conventionally been recovered as crude MDI and used as a raw material for polyurethane, but unless the reaction is carried out at high temperatures for a very long time (this involves thermal deterioration of the product). Since it is not possible to completely convert polycarbamate into polyisocyanate in a thermal decomposition reaction, as mentioned above, a small amount of high-boiling point polycarbamate raw materials and carbamate isocyanate reaction intermediates that cannot be separated and removed by distillation coexist in the bottom liquor. This is generally unavoidable. As a result, the quality as a polyurethane raw material was unsatisfactory, and root MDIL could not be obtained. In order to avoid this, as described above, it is necessary to find a reaction method that completes the thermal decomposition reaction in as short a time as possible and converts all the raw materials and intermediates into polyisocyanate.

According to the process of the invention, the high-boiling components obtained from the bottom of the distillation column are provided separately from the raw material pyrolysis reactor in order to achieve substantially complete conversion to polyisocyanate. Further pyrolysis takes place in a pyrolysis reactor. This thermal decomposition can also be carried out under the same reaction conditions and reaction method as the thermal decomposition of the raw materials. Unexpectedly, it has been found that by thermal decomposition of this high-boiling component, residual unreacted raw materials and intermediates containing carbamate groups can be virtually completely converted into polyisocyanates in a very short period of thermal decomposition. did.

Furthermore, in the method of the present invention, since unreacted raw materials and intermediate raw materials are re-thermally decomposed, there is no need to carry out the first thermal decomposition step of the raw materials under harsh conditions in order to ensure completeness of the reaction. That is, the first thermal decomposition step can be carried out in a shorter reaction time than conventionally. The re-thermal decomposition of the high-boiling components in the distillation column is preferably carried out at a higher temperature than the first thermal decomposition because the proportion of heavy substances is relatively high. For example, pyrolysis of high boiling components can be carried out at a temperature of 5 to 20°C higher than the pyrolysis temperature of the initial feedstock. The reaction time for this rethermal decomposition is selected so that unreacted polycarbamate and intermediate carbamate isocyanate are completely thermally decomposed to polyisocyanate, and varies depending on the reaction conditions, but is generally from several seconds to several minutes. Very short thermal decomposition, preferably within 1 minute, is sufficient.

The thermal decomposition of the above-mentioned high-boiling components can be carried out in a small reactor because the amount to be treated is small. For example, a thin film type falling reactor is particularly effective for thermal decomposition of heavy substances, and the reaction time is also very short. Therefore, deterioration of the polyisocyanate is avoided.

Further, this thermal decomposition reaction can be carried out either batchwise or continuously.

In the method of the present invention, by carrying out thermal decomposition twice in separate reactors as described above, it is possible to select the optimal reaction method and reaction conditions for each, and the polymer is produced in one thermal decomposition. Compared to the case where complete conversion to isocyanate is attempted, the total thermal decomposition reaction time is shorter, so thermal deterioration of the produced polyisocyanate due to concurrent side reactions is reduced, and a product with improved quality can be obtained. Note that it is possible to recirculate the high boiling point components to the raw material thermal decomposition reactor.
Although known for DI, such recycling methods do not provide the advantages mentioned above.

If appropriate conditions are selected, the product obtained by further thermally decomposing high-boiling components by the method of the present invention does not substantially contain unreacted polycarbamate and intermediate carbamate isocyanate, and therefore remains of high quality as it is. Root MD
It is recovered as a product equivalent to I (substantially consisting only of dinuclear and polynuclear polyisocyanates). Pure M ahead of this
It can also be mixed with DI in an appropriate ratio to form a product.

If this high-boiling product has a high MDI content, it may be separated again into pure MDI and crude MDI by distillation.

In recovering the thermal decomposition reaction product, it is preferable to completely remove the coexisting reaction solvent by distillation or the like. Distillation is preferably completed in as short a time as possible at a temperature as low as possible, for example at a temperature of 230° C. or lower, preferably at a temperature of 150° C. or lower, in order to suppress a decrease in yield and purity due to thermal alteration of the polyisocyanate product. .

The invention will now be illustrated by examples. In the examples, percentages are by weight unless otherwise specified.

Analysis in Examples was conducted as follows. Determination of phenyl carbamate, polycarbamate conversion, polyisocyanate yield, and polynuclear distribution was performed by taking a sample of the reaction product, diluting it with dry methanol and/or dry tetrahydrofuran, and then using high-performance liquid chromatography and gel chromatography. -Analyzed by permissicon chromatography. The NGO value of polyisocyanate is JI
It was determined according to the test method described in SK-1605 (1985).

Seed 1 spot 1 (Synthesis of polycarbamate by al condensation reaction: Into a 500-capacity flask equipped with a temperature needle, stirrer and dropping funnel, add 20 g of ethyl N-phenyl carbamate, 96%
24.7 g of sulfuric acid and 20 g of water were charged, and 4.9 g of a 37% formaldehyde aqueous solution (formalin) was added dropwise while maintaining the mixture at 50°C. After the dropwise addition was completed, the mixture was reacted for 2 hours on a 90° C. oil bath while stirring.

After the reaction was completed, the reaction solution was cooled to room temperature, 100 g of methyl ethyl ketone was added to completely dissolve the insoluble matter, and then the solution was separated. The aqueous layer was extracted again with 100 g of methyl ethyl ketone, the extracts were combined, 1001111 water was added to the methyl ethyl ketone layer, and 50% aqueous sodium hydroxide solution was added until the aqueous layer became neutral. After this neutralization, the methyl ethyl ketone layer was separated, thoroughly washed with pure water, and then methyl ethyl ketone and water were evaporated using a rotary evaporator to obtain 20 g of a residue. Analysis of this residue revealed that its composition was 1 unreacted N-phenyl carbamate.
2%, dinuclear polycarbamate (MDU) 63%, 3
18% of polycarbamate and 7% of other products were found to be more than the core.

Cation impurities in this condensation reaction product were removed by treatment with an ion exchange resin as follows. That is, 5 g of the polycarbamate product obtained above with the above composition was mixed with 10 g of methanol.
A glass column (diameter 20 mm x height 50 fi) packed with 50 g of strongly acidic cation exchange resin (Dowex 50W The mixture was passed at a flow rate to room temperature. After all the solution has passed through, 500 methanol is further passed through to flush out the polycarbamate remaining in the column, and combined with the above-mentioned passed through solution, 6
A processing liquid of 0.00 was obtained. Methanol was distilled off from this liquid to obtain 4.9 g of solid.

Mountain) Synthesis of polyisocyanate by pyrolysis: Stirrer,
50 equipped with a solvent supply pipe, nitrogen gas introduction pipe and discharge pipe
A 0 m glass reactor was installed in an oil bath, and a glass condenser (16 diameter x 2 height) was placed above the nitrogen gas discharge pipe.
001, filled with glass filler (approximately 172 in volume) was connected to a Liebig cooling tube, and a water bath and an ethanol/dry ice bath were placed further ahead. 4.9 g of the polycarbamate treated with an ion exchange resin was charged into this reactor along with 300 g of 1-chloronaphthalene as a solvent and 100 g of cyclohexylbenzene, the internal temperature was raised to 248°C, and dry nitrogen gas was blown in as a carrier. The thermal decomposition reaction was carried out for about 1 hour while stirring.

The final temperature reached was 250°C. During this time, the solvent lost due to evaporation was replenished so that the liquid level remained constant. The supply ratio of 1-chloronaphthalene and cyclohexylbenzene is
Initially, the ratio was 3/1, and at the end of the reaction, it was changed to only 1-chloronaphthalene. Dry nitrogen gas is 100
It was preheated to 0.degree. C. and blown into the solution at a rate of 0.degree./min. The evaporated components were cooled to 25° C. or lower using the cooler described above to recover the solvent, and the by-produced ethanol was collected in an ethanol/dry ice bath, and the amount produced was measured by gas chromatography.

The analysis results of the obtained thermal decomposition reaction product were as follows. The conversion rate of binuclear units (MDU) in the raw material polycarbamate was 99.6%, and the conversion rate of binuclear units (MDU) in the polyisocyanate produced was 99.6%.
The yield of MDI) is 93.0 based on the MDU of the charge.
Mol%, yield of intermediate methylene diphenyl monoethyl carbamate monoisocyanate (MMI) is 3.2
It was mol%. Moreover, the amount of ethanol recovered was 94.0% of the theoretical amount.

128.1 g of the obtained thermal decomposition reaction product was distilled under reduced pressure at 90°C and 0.8 mmFlg using a rotary vane type thin film evaporator to obtain 100.1 g of a liquid containing 3.9% MDI as a distillate.
5 g was obtained. The distillation residue has an MDI of 42.3% and an MMI of 5.
．． It was 28.2 g of a liquid containing 2%, MDUl, and 2%.

The solvent was removed from the former distillate under reduced pressure, and the MDI was 3.9.
g was collected. No impurities other than isomers were observed in this MDI, and it had the quality of pure MDI.

The brown liquid containing high boiling point substances from the distillation residue is collected in a falling reactor (diameter 20 m, height 200 m) filled with glass beads.
was supplied by a pump to cause a thermal decomposition reaction at a temperature of 260°C. The residence time of most of the liquid was 3 seconds, and during the reaction, preheated dry nitrogen gas was supplied from below as a carrier at a flow rate of 100 w1/min. The flowing liquid was collected in a pool at the bottom of the reactor. Low-boiling substances such as solvents were removed from the system using nitrogen. The remaining solvent was removed from the collected reaction product under reduced pressure to obtain 50 g of a viscous liquid. The reaction product is
As a result of the analysis, it consisted of 82% MDI, 10% trinuclear or higher polyisocyanate, 8% other compounds, MDU,
MMI and polynuclear polycarbamates were not detected by liquid chromatography. That is, by thermal decomposition of this distillation residue, the carbamate groups in the unreacted raw materials and intermediates were completely thermally decomposed into isocyanate groups, and a polyisocyanate product containing no carbamate bodies was obtained. The NCO value was 33.2 for MDI obtained from the distillate and 30.1 for the polyisocyanate obtained from thermal decomposition of the distillation residue.

(Effects of the Invention) The following effects are achieved by the present invention. That is, (1) a polyisocyanate product substantially free of carbamate group-containing compounds can be obtained, and crude MD
A product of excellent quality can be obtained.

(2) Since the reaction can be completed in a shorter thermal decomposition reaction time than before, thermal loss of the main product MDI is minimized and the yield of pure MDI is high.

■ By performing pyrolysis in two stages, pyrolysis can be carried out according to the properties of the raw material, and reaction conditions and equipment can be appropriately selected for each pyrolysis, reducing utility costs and equipment costs. is planned.

(2) The thermal decomposition reaction can be carried out at normal pressure without using a catalyst, eliminating the need for post-treatment to remove the catalyst, and allowing the reaction to be carried out with a simple device.

As a result, according to the present invention, a polyisocyanate having a composition and physical properties suitable as a raw material for producing polyurethane can be economically advantageously produced.

Claims (4)

[Claims] (1) In the method for producing polymethylene polyphenyl polyisocyanate by liquid-phase pyrolysis of polymethylene polyphenyl polycarbamate, the obtained pyrolysis reaction product is distilled to substantially convert the reaction product into a dinuclear form. It is separated into a low-boiling point component consisting of methylene diphenyl diisocyanate and a high-boiling point component containing a polynuclear polyisocyanate having trinuclear bodies or more in addition to binuclear bodies, unreacted polycarbamate raw materials, and an intermediate carbamate isocyanate compound. Production of polymethylene polyphenyl polyisocyanate characterized by further liquid-phase pyrolysis of high-boiling components discharged from the bottom in a reactor provided separately from the pyrolysis reactor for raw material polymethylene polyphenyl polycarbamate. Method. (2) The method according to claim 1, wherein the thermal decomposition is performed at normal pressure in the absence of a catalyst. (3) The method according to claim 1 or 2, wherein the high boiling point component is thermally decomposed at a higher temperature and in a shorter time than the thermal decomposition of the raw material. (4) The method according to claim 3, wherein the thermal decomposition of the high boiling point component is carried out in a falling reactor.