HK1072932B - Production of mixtures of diisocyanates and polyisocyanates from the diphenylmethane series - Google Patents

Description

Preparation of mixtures of diisocyanates and polyisocyanates of the diphenylmethane series

Technical Field

The invention relates to a method for producing mixtures of diisocyanates and polyisocyanates of the diphenylmethane series having a high content of 4, 4 '-and 2, 4' -methylenediphenyl diisocyanate, and to the use of such mixtures for producing polymers.

Background

It is known that diisocyanates and polyisocyanates (MDI) from the diphenylmethane series can be prepared by phosgenation of the corresponding diamines and polyamines (MDA) from the diphenylmethane series. The corresponding diamines and polyamines from the diphenylmethane series can themselves be prepared by condensation of aniline and formaldehyde. The corresponding diisocyanates 2, 2 ' -MDI, 2, 4 ' -MDI and 4, 4 ' -MDI, known to the person skilled in the art as binuclear MDI (diisocyanate of the diphenylmethane series), are obtained by phosgenating diamines from the diphenylmethane series. However, in the condensation of aniline and formaldehyde, the binuclear MDA (methylene diphenyl diamine MDA) is further reacted with formaldehyde and aniline to produce a more polynuclear MDA grade, which, after the phosgenation, gives rise to polynuclear species in the polymeric MDI (polyisocyanates of the diphenylmethane series). For many practical preparative applications, it is preferred to produce a high proportion of binuclear MDI. This can be achieved in two different ways according to the current state of the art.

1. A large excess of aniline is used in the acid-catalyzed condensation of aniline and formaldehyde, and the excess aniline is separated from the reaction mixture and recycled. In the condensation reaction, a large excess of aniline produces an MDA mixture with a high proportion of dual core MDA. The higher the aniline/formaldehyde ratio used in the reaction, the higher the content of dinuclear content contained in the reaction product. Furthermore, the ratio of 2, 4 '-MDA to 4, 4' -MDA is also affected by the concentration of the acid catalyst. High catalyst concentrations are beneficial for 4, 4' -MDA. Low concentrations are beneficial for 2, 4' -MDA. (see, for example, EP-158059-B1 and EP-3303-B1). Such MDA grades are referred to hereinafter as polymer a or polymer B grades for MDI compositions comprising a low 2, 4 '-isomer content or a high 2, 4' -isomer content, respectively. The desired MDI compositions may be prepared by phosgenation of a previously selectively prepared MDA stage.

2. Polymeric MDA is prepared from aniline and formaldehyde using an acid catalyst in a conventional manner. The polymeric MDA undergoes a phosgenation reaction and is separated by a distillation process into a monomer-rich and a polymer-rich fraction. The polymeric MDI fractions may be used as commercial polymeric MDI products. According to the prior art, the monomeric MDI fraction is separated by distillation or crystallization into the isomers 4, 4 ' -MDI and a mixture of about 50% 2, 4 ' -MDI and 50% 4, 4 ' -MDI. The product of the two monomers can be sold or further processed to produce a mixed product having a high content of binuclear MDI and a suitable isomer ratio of polymeric MDI.

In both cases, the investment and energy consumption involved indicate the high cost of producing MDI mixtures with a high proportion of binuclear MDI.

In the first case, in the condensation reaction of aniline and formaldehyde, the aniline in large excess must be recycled, separated from the reaction mixture by distillation and recycled.

In the second case, the condensation of aniline and formaldehyde with a small aniline excess, in addition to the desired diamines, produces a significant amount of polynuclear MDA stage, which is then phosgenated together with diamines. In order to prepare diisocyanates from mixtures of diisocyanates and polyisocyanates (binuclear MDI and polynuclear MDI grades), the diisocyanates have to be distilled off from the mixtures of diisocyanates and polyisocyanates (monomer/polymer separation). After separation of such monomers, the isomers have to be separated from each other by distillation and/or crystallization, which involves the use of large amounts of equipment and high energy consumption. The laboriously separated isomers are then remixed to the high monomer content final product.

However, in the prior art processes, the crude monomer fraction obtained by the monomer/polymer separation (crude monomer, consisting essentially of diisocyanate) still contains undesired high concentrations of secondary components. For example, such crude MDI monomer mixtures contain many times the maximum concentration of 50ppm chlorobenzene and the maximum of 20ppm phenyl isocyanate (preferably the maximum of 20ppm chlorobenzene and the maximum of 10ppm phenyl isocyanate normally required by polyurethane manufacturers). Furthermore, the usual crude MDI monomer mixtures contain significant amounts of uretdione (uretdione) (dipenta-MDI) which, at higher concentrations, leads to product cloudiness and to solid precipitates in the MDI. The diisocyanates distilled off in the monomer/polymer separation must therefore be free of secondary components in a subsequent distillation stage involving considerable cost and effort.

In other words, specific MDI grades having specific isomer (2, 2 ' -MDI, 2, 4 ' -MDI and 4, 4 ' -MDI) contents are commercially available. In the prior art, therefore, compositions corresponding to commercial MDI grades with low contents of secondary components are produced by complex, multistep isomer separations by means of distillation and/or crystallization. The isocyanate compositions required for a particular application are then prepared by blending these MDI grades.

However, for many applications of MDI grade, the polymer manufacture does not require a high 4, 4 '-MDI purity, in other words, for example, a low 2, 2' -MDI content in the raw materials to be blended. The use of pure 4, 4 '-MDI grades, which are obtained by means of several complicated distillation stages and have a very low 2, 2' -MDI content, in combination with other MDI grades to obtain isocyanate compositions suitable for specific applications is therefore unsatisfactory from the energy consumption point of view.

The following are also known from the prior art:

for example, the following two documents describe the direct production of mixed MDI products by means of selective MDA synthesis. Keggenhoff, Maehlmann & Eiffer (EP-158059B1) produced an MDA mixture with a high content of 4, 4 ' -MDA, comprising about 80% of 4, 4 ' -MDA and about 10% of 2, 4 ' -MDA, and a dual core content of about 90%. In contrast, Eiffer & Ellendt (EP-3303B1) produces MDA with a high monomer content with a dinuclear content of 88%, comprising 19% 2, 2 ' -MDA, 36% 2, 4 ' -MDA and 45% 4, 4 ' -MDA. Molar ratios of aniline/formaldehyde of greater than 8 are required to produce these products, which means that large amounts of aniline must be recycled. Furthermore, for many of these grades, high consumption of hydrochloric acid (catalyst) and NaOH (neutralization catalyst) is necessary. In particular, products with a high content of 4, 4' -MDA indicate a high consumption of the particular hydrochloric acid catalyst. In particular, the preparation of MDA grades with high contents of 2, 4 '-MDI and high monomer content tends to result in high, often undesirable, minor components of 2, 2' -MDI. In many applications, high concentrations of 2, 2' -MDI are undesirable due to their low reactivity.

The preparation of polymeric and monomeric MDI products is generally known in the literature. Polymeric MDA with a dinuclear content of 46 to 65% can be prepared according to the methods described in DE-2750975A1 or DE-2517301A 1. Both distillative (DE-3145010A1) and crystalline (EP-A-482490) processes which are used primarily for the industrial separation of isomers from MDI monomers are specified in said document. However, none of the documents describes the direct use of a crude monomeric MDI fraction from a monomer/polymer separation process as a source of feedstock for MDI blending. Rather, the person skilled in the art is working on the most economical preparation methods to prepare ultrapure monomeric isomer mixtures comprising more than 98% of 4, 4 '-MDI and mixtures of 2, 2' -MDI and 4, 4 '-MDI each in an amount of about 50%, which may optionally comprise up to 2.5% of 2, 2' -MDI (M.stepanski, P.Faesser: "New moisture process for purification and analysis of MDI gauges" Sulzer Chemtech, present at the polyurethane conference 2002 in Salt Lake City, 10/2002).

In the polyurethane industry, it is common to prepare MDI mixtures with high monomer content by blending a polymeric MDI product with a monomer product that is repeatedly distilled. This process is used to prepare low viscosity blended MDI products, for example in US-A-5,258,417.

The purification of crude polymeric MDI mixtures has in principle been investigated in a number of documents. For example, chemical additives are used to remove impurities (U.S. Pat. No. 4, 3,925,437, U.S. Pat. No. 3,3,793,362, DE-A-2316028). In DD-A-118, 105 and GB-A-1, 114,690, solvents were added in an attempt to remove chemically bound impurities. DD-A-271,820 mentions desorption of MDI and TDI (toluene diisocyanate), which results in significant decomposition of 0.1 to 10% by weight of the starting materials, but essentially achieves the removal of the chemically bonded impurities which are difficult to reconcile.

US-A-3,857,871 discloses A desorption process for polymeric MDI which results in A reduction in acidity and hydrolysable chlorine in the product. GB-A-1,362,708 describes a process for purifying polymeric MDI which apparently also significantly reduces the amount of hydrolysable chlorine. However, the product still contained 0.1% solvent.

Disclosure of Invention

It is therefore an object of the present invention to provide a simple and energy-saving process for preparing MDI mixtures having a high monomer content and to provide a process for preparing MDI mixtures having the desired MDI isomer ratio which comprise very low proportions of secondary components, in particular solvents, phenyl isocyanate, uretdione and phosgene.

This object is achieved by separating in a separate distillation step a fraction which contains at least 95% by weight of binuclear methylene diphenyl diisocyanate, which contains more than 60% by weight of 4, 4 ' -MDI, from 4 to 35% by weight of 2, 4 ' -MDI and from 0.1 to 10% by weight of 2, 2 ' -MDI, and a maximum of 20ppm phenyl isocyanate from a mixture of crude diisocyanates and polyisocyanates.

Drawings

FIG. 1 is a graphical representation of the binuclear content of some of the commercial feedstocks from which MDI blends are made.

FIG. 2 is a graphical representation of the dual core content of some blended MDI products.

Detailed Description

The invention relates to a method for producing a diphenylmethane diisocyanate fraction containing at least 95 wt.% binuclear methylene diphenyl diisocyanate, wherein,

a) reacting aniline and formaldehyde in the presence of an acid catalyst to obtain diphenylmethanediamine and polyamine containing binuclear methylenediphenyldiamine,

b) phosgenating said di-and polyamines of the diphenylmethane series comprising dinuclear methylenediphenyldiamine, optionally in the presence of a solvent, to obtain a crude di-and polyisocyanate,

c) separating a fraction comprising at least 95% by weight dinuclear methylene diphenyl diisocyanate and a maximum of 20ppm of phenyl isocyanate from the crude diisocyanate and polyisocyanate in a separate distillation step; the binuclear methylene diphenyl diisocyanate has 60% by weight or more of 4, 4 ' -MDI, 4 to 35% by weight of 2, 4 ' -MDI, and 0.01 to 10% by weight of 2, 2 ' -MDI, based on the mass of the fraction; in a preferred embodiment, the separate distillation step is provided with an optional upstream and/or downstream separation of low-boiling components; in another preferred embodiment, the fraction optionally comprises a maximum of 50ppm of solvent.

In a preferred embodiment, low boilers are removed before and/or after the separation of the fraction in step c).

In step C), a fraction comprising at least 99% by weight of binuclear methylene diphenyl diisocyanate and a maximum of 10ppm of phenyl isocyanate and optionally a maximum of 20ppm of solvent is preferably separated from the crude diisocyanate and polyisocyanate; the above binuclear methylene diphenyl diisocyanate has 76% by weight or more of 4, 4 ' -MDI, 5 to 22% by weight of 2, 4 ' -MDI, and 0.2 to 3% by weight of 2, 2 ' -MDI, based on the mass of the fraction.

The maximum concentrations of phenyl isocyanate and solvent are calculated on the mass of the entire fraction separated.

Separating the distillate in a separate distillation step means that only the separated MDI is completely evaporated in the separate distillation step and condensed again, obtained as a distillate in another part of the distillation column. In contrast, in the optional upstream and/or downstream separation of the low-boiling components, the predominantly low-boiling components are separated by evaporation and subsequent condensation. Most MDI does not evaporate when separating the low boiling components.

Thus, separating the fraction in a separate distillation step also means that, in particular in the process according to the invention, after separating the fraction from the crude diisocyanate and polyisocyanate in a separate distillation step, the isomerized MDI diisocyanate is not substantially separated again.

By means of the process according to the invention, MDI mixtures having a high monomer content and a dual MDI content of at least 95% by weight, based on the mass of the fractions, and a low proportion of secondary components can be obtained with a significantly lower energy consumption than in the separation processes of the prior art. The energy consumption is reduced here compared to the processes of the prior art, since the condensation reaction of aniline and formaldehyde is avoided using a large excess of aniline, which is subsequently distilled and recycled. At the same time, however, by the process of the invention, in the distillation of the mixture of diisocyanates and polyisocyanates obtained by phosgenation, only one distillation reduces the amount of undesired secondary components and no subsequent distillation step is required. In the process of the invention, the separation of isomers by means of multiple distillations is avoided, while undesired secondary components are removed (according to the prior art).

In the prior art distillation of MDI isomers, considerable technical effort is also required to destroy trace components such as hydrolyzable chlorine components. However, this trace component is not a harmful substance in high monomer content MDI mixtures. To remove the residual trace components, the monomeric MDI is typically evaporated and cooled about 4-6 times during the isomer distillation before it is sold as pure 4, 4' -MDI. In contrast, the energy consumption for the preparation of crude monomeric MDI with a low solvent content according to the invention is much lower, since MDI is prepared in a separate distillation step with optional upstream and/or downstream separation of the low-boiling components.

By removing the troublesome low-boiling secondary components, such as phenyl isocyanate and optionally solvents, in the monomer/polymer separation stage, the process according to the invention makes it possible to obtain a high monomer content MDI fraction having a high binuclear MDI content of at least 95% by weight, based on the mass of the fraction. In step c), the separation of the separate distillation step is preferably carried out at an absolute pressure (top of the distillation column) of from 1 to 50 mbar and a temperature of 100 ℃ and 300 ℃. The removal of the minor components during the preparation of the distillate comprising at least 95% by weight binuclear methylene diphenyl diisocyanate and a maximum of 20ppm phenyl isocyanate and optionally a maximum of 50ppm solvent can be carried out by desorbing the crude MDI monomer mixture in a distillation (monomer/polymer separation) or by removing the low-boiling components before the actual body/polymer separation.

Desorption of the crude MDI monomer mixture obtained in the distillation (monomer/polymer separation) process involves separating off the low-boiling components comprising solvent, phenyl isocyanate and, for example, small amounts of residual phosgene, with a small substream of the MDI isomer at the top of the desorption column. The separated low boiling fraction may be returned to the MDI/solvent separation stage or preferably passed to a separate low boiling component separation column, freed from solvent and phenyl isocyanate, and recycled as additional MDI feed or used separately.

If the separation of the low-boiling components by desorption is dispensed with, the monomer/polymer separation stage can be designed as a distillation column with a side stream (side stream) draw-off. Here, the clean MDI monomer mixture may be separated in a side stream of the distillation column, with the low boiling components being separated at the top of the distillation column. The low-boiling components may be recycled or, preferably, freed from solvent and phenyl isocyanate in a separate low-boiling component separation column.

It is also possible to modify the removal of the low-boiling components so that phenyl isocyanate or solvent is virtually not passed to the monomer/polymer separation stage. This may be achieved, for example, by returning the distillate from the optional multi-step solvent removal process to the first solvent separation stage prior to the monomer/polymer separation step.

Crystallization is also a suitable method for purifying crude MDI monomer mixtures.

In this process, the product produced by the distillation column via the monomer/polymer separation step is preferably rapidly quenched to 35-80 ℃ to reduce the uretdione content. Suitable methods for quenching the continuous MDI monomer stream are already known in the art. For example, a recycle system with a recycle pump and a heat exchanger such as a 60 ℃ is adapted to cool the hot product from about 140 ℃ to 200 ℃ to 60 ℃ in a minimum amount of time, thereby minimizing uretdione formation.

The invention also relates to a process for preparing mixtures containing diisocyanates from the diphenylmethane series, in which:

a) preparing a fraction comprising at least 95% by weight of binuclear methylene diphenyl diisocyanate having more than 60% by weight of 4, 4 ' -MDI, 4 to 35% by weight of 2, 4 ' -MDI and 0.01 to 10% by weight of 2, 2 ' -MDI, and up to 20ppm phenyl isocyanate and optionally up to 50ppm solvent by the above process,

b) blending the fraction obtained in step a) with one or more mixtures comprising diphenylmethane diisocyanates and/or polyisocyanates.

The diphenylmethane diisocyanate fraction obtained by the process of the present invention, which contains at least 95% by weight of binuclear MDI, can be used for blending with polynuclear (polymeric) MDI and commercial MDI grades to obtain any MDI mixture in which the proportions of binuclear MDI and polynuclear MDI are freely adjustable and the proportions of the various MDI isomers are freely adjustable.

The diphenylmethane diisocyanate fraction obtained by the process of the present invention has preferably the following composition:

total binuclear content: more than or equal to 95 weight percent;

4, 4' -MDI content: > 60% by weight, based on the total amount of the fractions;

2, 4' -MDI content: 4-35 wt.%, based on the total amount of the distillate;

2, 2' -MDI content: 0.01 to 10 wt.%, based on the total amount of the fractions;

solvent content: 0-50ppm based on total amount of distillate;

phenyl isocyanate content: 0-20ppm based on total amount of distillate;

uretdione content: 0 to 0.5 wt.%, based on the total amount of the distillate.

Particularly preferred fractions have the following composition:

total binuclear content: not less than 99 wt%;

4, 4' -MDI content: > 76% by weight, based on the total amount of the fractions;

2, 4' -MDI content: 5-22 wt.%, based on the total amount of the distillate;

2, 2' -MDI content: 0.2 to 3 wt.%, based on the total amount of the distillate;

solvent content: 0-20ppm based on total amount of distillate;

phenyl isocyanate content: 0-10ppm based on the total amount of the fraction;

uretdione content: 0 to 0.1 wt.%, based on the total amount of the distillate.

The diphenylmethane diisocyanate fraction obtained by the process of the present invention, which contains at least 95% by weight of binuclear MDI, may be blended with polyisocyanates or other organic isocyanates. FIG. 1 provides a schematic representation of a commercial raw material product for preparing MDI blends. Figure 2 shows some examples of MDI-based blended products in the same parameter ranges. The horizontal axis represents the binuclear total content, and the vertical axis represents the ortho-binuclear total content (2, 2 '-MDI +2, 4' -MDI). The dashed lines form triangles in which blends can be prepared from a particular raw product.

The diphenylmethane diisocyanate fraction obtained by the process according to the invention, which contains at least 95% by weight of binuclear MDI, may be blended with one or more mixtures comprising aromatic isocyanates. In addition to the aforementioned mixtures comprising diphenylmethane diisocyanates and/or polyisocyanates, it is preferred to use mixtures comprising toluene diisocyanates (e.g., 2, 4-TDI, 2, 6-TDI) or naphthalene diisocyanates (e.g., 1, 5-NDI) or mixtures of these isocyanates. In principle, however, it is also possible to use other aromatic and/or aliphatic mono-, di-, tri-or polyfunctional isocyanates for the incorporation.

The diphenylmethane diisocyanate fractions obtained by the process according to the invention and containing at least 95% by weight of binuclear MDI and the blend products prepared therefrom can be used in conventional isocyanate-modification reactions, such as carbodiimidization and uretonimine synthesis, or in internal OH-functional polyesters, C₂、C₃And/or C₄Polyethers, uretdiones, allophanates, biurets and/or urea derivatives.

The diphenylmethane diisocyanate fraction obtained by the process according to the invention, which contains at least 95% by weight of binuclear MDI, can be mixed as a mixing component with other MDI-based diisocyanates and polyisocyanates and/or with other isocyanates used for preparing the isocyanate component for preparing polymers. The final part obtained from these blended products covers the entire field of polyurethane chemistry. Examples are as follows:

rigid foams for the insulation and refrigeration industry

Packaging foams

Flexible foams and flexible molded foams for the furniture and vehicle industry

Coatings, adhesives, sealants and elastomer applications

Semi-rigid polyurethane foams

The present invention allows the preparation of blended MDI products using much less equipment and with reduced energy consumption when distilling in the MDI isomer separation stage or aniline distillation in the MDA stage. At the same time, if the use of polymer a or polymer B grades, which were prepared in the prior art using large amounts of hydrochloric acid (controlling the isomer distribution in MDA) and NaOH, could be avoided, the average hydrochloric acid and NaOH feed consumption would be significantly reduced in the MDA stage.

In many blended products, the 2, 2 '-MDI content can also be reduced if the polymer B-grades having a high 2, 2' -MDI content are no longer prepared directly and are used as mixing components for end products having a high monomer content. The use of MDI having a low 2, 2 ' -MDI content in the blend, such as the low 2, 2 ' -MDI content fraction obtained by the process of the present invention, allows the preparation of MDI products having a significantly lower 2, 2 ' -MDI content and improved reactivity by blending. The polymers made from them also exhibit improved polymer properties.

Examples

The process for solving the object of the present invention is illustrated with reference to the following examples. All% values are in weight percent (wt%).

Commercially available MDI mixtures (MCB ═ chlorobenzene solvent, PHI ═ phenyl isocyanate) can be prepared using MDI starting materials of the composition described below:

rank of	2，2’-MDI	2，4’-MDI	4，4’-MDI	Total amount of binuclear	MCB	PHI
							Polymer grade (200mPas)	<0.2％	3％	39％	42％	<10ppm	<8ppm
Pure 4, 4' -grade	0％	1.5％	98.5％	100％	<10ppm	<8ppm
							Rich in 2, 4' -stage	<2％	55％	44％	100％	<10ppm	<8ppm
Rich in 2, 2' -stage	50％	50％	0％	100％	<10ppm	<8ppm
							Polymer class A	0.7％	11％	78％	90％	<10ppm	<8ppm
Polymer B grade	6％	31％	51％	88％	<10ppm	<8ppm
							Crude monomer	0.7％	11％	88％	>99％	250ppm	44ppm
Pure crude monomer	0.7％	11％	88％	>99.5％	15ppm	8ppm

Fraction obtained by the method of the invention

The two polymer grades a and B are prepared using a high excess of aniline, thus requiring a complex and expensive distillation and recycling of the aniline. In another aspect, the MDI isomer distillate pure 4, 4 ' -, 2, 4 ' -rich and 2, 2 ' -rich fractions may be prepared as follows: the mixtures of diisocyanates and polyisocyanates obtained by phosgenation are subjected to extremely complex, multistage distillations in order to separate off the undesirable secondary components and to establish the desired specification for the individual binuclear MDI isomers (2, 4 ' -MDI, 2 ' -MDI and 4, 4 ' -MDI).

On the other hand, polymer grades and crude monomer grades (crude monomers and pure crude monomers) can be obtained from the mixtures of diisocyanates and polyisocyanates obtained by phosgenation by means of separate distillation with little effort and low energy consumption. Here, the polymer grade is prepared as a bottom product and it contains a high proportion of polynuclear MDI. The crude monomer stage is withdrawn at the top of the distillation column or as a side stream. Crude monomers differ from pure crude monomers in that they are obtained by the process according to the invention and therefore have only a very low residual proportion of secondary components, such as phenyl isocyanate and optionally solvent, and therefore no additional work-up by distillation is required.

Example 1 (comparative)

The mixed MDI product produced from 17 wt.% polymer grade, 51 wt.% polymer grade a and 32 wt.% polymer grade B had the following product quality (MCB ═ chlorobenzene as solvent):

rank of	2，2’-MDI	2，4’-MDI	4，4’-MDI	Total amount of binuclear MDI	MCB
						Blend 1	3.9％	16.3％	62.5％	82.7％	<10ppm

Although the rectified products (pure 4, 4 ' -grade, 2 ' -rich grade, 2, 4 ' -rich grade) cannot be used to prepare blend 1, 83 wt.% of polymer grade a and/or B (prepared using a very complicated process) must be used. Thus, the preparation of blend 1 in example 1 is very energy intensive.

Example 2 (comparative)

The mixed MDI product made from 36.4 wt% polymer grade, 37.1% 4, 4 '-grade, and 26.5% 2, 4' -grade had the following product qualities:

rank of	2，2’-MDI	2，4’-MDI	4，4’-MDI	Total amount of binuclear MDI	MCB
						Blend 2	0.3％	16.3％	62.5％	79.1％	<10ppm

Although the preparation of blend 2 did not require any elaborate preparation of polymer MDA grades A or B, or phosgenation products (polymers A and B), 64% by weight pure 4, 4 '-and 2, 4' -rich grades (prepared using a very complicated process) had to be used. Thus, the preparation of blend 2 in example 2 is very energy intensive. But the 2, 2' -MDI content was significantly reduced compared to blend 1.

Example 3 (comparative)

The mixed MDI product made from 35.9 wt% polymer grade, 45.7% crude monomer, 18.3% 2, 4' -grade had the following product quality:

rank of	2，2’-MDI	2，4’-MDI	4，4’-MDI	Total amount of binuclear MDI	MCB	PHI
							Blend 3	0.56％	16.3％	62.5％	79.4％	114ppm	22ppm

Example 3 shows that blend 3 does not meet the customer's requirements, i.e., the solvent content is less than 20ppm and the PHI content is less than 10ppm (blend 2 meets these). However, the low 2, 2' -MDI content is comparable to that obtained in example 2.

Example 4 (inventive)

The mixed MDI product made from 35.9 wt% polymer grade, 45.7% pure crude monomer, 18.3% 2, 4' -grade had the following product quality:

rank of	2，2’-MDI	2，4’-MDI	4，4’-MDI	Total amount of binuclear MDI	MCB	PHI
							Blend 4	0.56％	16.3％	62.5％	79.4％	<10ppm	<8ppm

Only about 18% of the highly energy consuming rectified isomer product is required to make this blend, which means that the amount of rectified isomer product is reduced by about 70%. The two polymer grades A and B produced from MDA using very complicated processes can be dispensed with altogether. A product was obtained whose quality was comparable to that of example 2, with only a slight increase in the 2, 2' -MDI content.

Example 5 (inventive)

If a 2, 2 '-MDI content of up to 3.9% by weight is actually desired, an additional product quality (2, 2' -rich type) comprising about 50% 2, 2 '-MDI and about 50% 2, 4' -MDI can be used as the fourth component. The starting value of example 1 can be reproduced accurately in this way.

The mixed MDI product made from 30.2% polymer grade, 51.8% pure crude monomer, 11.4% 2, 4 '-grade and 6.7% 2, 2' -grade had the following product qualities:

rank of	2，2’-MDI	2，4’-MDI	4，4’-MDI	Total amount of binuclear MDI	MCB	PHI
							Blend 5	3.9％	16.3％	62.5％	82.7％	<10ppm	<8ppm

Only about 18% of the rectified product (2, 2 '-rich stage and 2, 4' -rich stage) is required to make blend 5. The elaborated polymer grades a and B can be omitted together. This results in a reduction of the amount of raw material produced by the high energy consumption by about 80% compared to example 1.

Example 6 (comparative)

1000g of aniline and 306g of 31.9% aqueous hydrochloric acid are mixed in a stirred tank reactor at 40 ℃. 480g of 32% formaldehyde solution were added dropwise over 15 minutes. First, it was stirred at 40 ℃ for a further 15 minutes and the temperature was slowly raised to 100 ℃ over the next 2.5 hours. The reaction mixture was then stirred at reflux for 10 hours at 100 ℃ and neutralized with 50% aqueous sodium hydroxide, the aqueous phase was separated and the organic phase was washed with water. The organic solution was removed and the excess aniline was distilled off in vacuo. The MDA reaction product was poured into a second stirred tank reactor containing a 15% ice-cooled solution of phosgene in chlorobenzene (MCB), with a 150% molar excess of phosgene. The reaction solution was slowly heated to 100 ℃ over 1 hour while 40 liters/hour of phosgene was continuously added. The mixture was heated to boiling point over a further 1 hour period, phosgene addition was stopped and a vacuum was applied. The temperature was gradually increased to 210 ℃, the pressure was reduced to 3 mbar, and the solvent was completely removed. A crude MDI mixture was prepared comprising 58 wt.% monomeric MDI (comprising 0.21 wt.% 2, 2 ' -MDI, 5.1 wt.% 2, 4 ' -MDI, 52.7 wt.% 4, 4 ' -MDI), 65ppm MCB and 14.5ppm PHI.

The crude MDI mixture is separated into a polymeric MDI product and a crude monomer fraction (binuclear MDI). A glass column with product fed to the bottom evaporator was used as experimental setup, the column containing only the droplet separator packing and no separation plate. The distillate was completely condensed at the top and removed. The top pressure was adjusted to 5 mbar by means of a vacuum pump.

A stream of 500g/h of crude MDI mixture was fed to the continuously operated apparatus at 180 ℃. After 2 hours of feeding, the following streams were discharged from the apparatus as products:

bottom of distillation apparatus: 62g in 10 minutes, having the following composition,

0.09％2，2’-MDI、3.7％2，4’-MDI、39.8％4，4’-MDI、0.5ppm MCB、4ppm PHI

the rest is as follows: polymeric MDI (lower bottom component viscosity at 25 ℃ 185mPas)

Distillation apparatus top stream: 21g in 10 minutes, having the following composition,

all% of the 0.57% 2, 2 ' -MDI, 9.0% 2, 4 ' -MDI, 89.3% 4, 4 ' -MDI, 241ppm MCB, 44ppm PHI composition are by weight of each complete sample.

Example 7 (invention):

the crude MDI mixture of example 6 was separated into polymeric MDI product and clean pure crude monomers as well as low boiling components.

The experimental setup used was as follows: a glass column with a product feeder feeding the bottom evaporator and stainless steel distillation packing with 4 theoretical separation steps, the fluid reflux from the top of the experimental column can be withdrawn with the reflux portion back into the column. At the top, the distillate can be completely condensed and 95% of it is returned to the column as reflux using a sample separator. The top pressure was set to 5 mbar by means of a vacuum pump.

A stream of 500g/h of crude MDI mixture was fed to the continuously operated apparatus at 180 ℃. After 2 hours of feeding, the following streams were discharged from the plant as products:

bottom of distillation apparatus: 60g in 10 minutes, having the following composition,

0.08％2，2’-MDI、3.6％2，4’-MDI、39.9％4，4’-MDI、0.5ppm MCB、4ppm PHI

the rest is as follows: polymeric MDI (bottom component viscosity at 25 ℃ C: 205mPas)

Distillation apparatus top stream: 2g in 10 minutes, having the following composition,

2.0％2，2’-MDI、17.0％2，4’-MDI、80.7％4，4’-MDI、730ppm MCB、380ppm PHI

side stream of the distillation apparatus between two separating packing elements (corresponding to pure crude monomer fraction): 19g in 10 minutes, with a composition of 0.45% 2, 2 ' -MDI, 8.7% 2, 4 ' -MDI, 90.7% 4, 4 ' -MDI, 12ppm MCB, 6ppm PHI

All% in the composition are by weight of each complete sample.

The above examples show that the preparation of a blended product from pure crude monomers obtained by the process of the invention provides decisive advantages over the prior art:

by replacing polymer grades a and B in the blend with pure crude monomer, the 2, 2' -MDI content in the blended product can be significantly reduced, which increases the reactivity and reduces the residual monomer content.

By replacing the crude monomers in the blend with pure crude monomers, the content of solvents such as MCB, phenyl isocyanate and phosgene in the blended product can be significantly reduced.

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims (6)

1. A process for preparing a diphenylmethane-series diisocyanate fraction comprising at least 95% by weight dinuclear methylene diphenyl diisocyanate, said process comprising:

a) reacting aniline with formaldehyde in the presence of an acid catalyst to obtain diphenylmethanediamine and polyamine containing binuclear methylenediphenyldiamine,

b) phosgenating di-and polyamines of the diphenylmethane series containing binuclear methylene diphenyl diamine to obtain crude di-and polyisocyanates,

c) separating a fraction comprising at least 95% by weight dinuclear methylene diphenyl diisocyanate and not more than 20ppm of phenyl isocyanate from the crude diisocyanate and polyisocyanate in a separate distillation step; the binuclear methylene diphenyl diisocyanate includes 60% by weight or more of 4, 4 ' -methylene diphenyl diisocyanate, 4 to 35% by weight of 2, 4 ' -methylene diphenyl diisocyanate, and 0.01 to 10% by weight of 2.2 ' -methylene diphenyl diisocyanate, based on the mass of the fraction.

2. The process of claim 1, wherein step b) is carried out in the presence of a solvent.

3. The process of claim 2, wherein no more than 50ppm of solvent is present in the fraction separated in step c).

4. The process according to claim 1, characterized in that low boilers are removed before and/or after the separation of the fraction in step c).

5. The process according to claim 1, characterized in that in step c) a fraction comprising at least 99% by weight dinuclear methylene diphenyl diisocyanate and a maximum of 10ppm phenyl isocyanate is separated; the binuclear methylene diphenyl diisocyanate comprises 76% by weight or more of 4, 4 ' -methylene diphenyl diisocyanate, 5 to 22% by weight of 2, 4 ' -methylene diphenyl diisocyanate, and 0.2 to 3% by weight of 2, 2 ' -methylene diphenyl diisocyanate, based on the mass of the fraction.

6. The process of claim 2, wherein no more than 20ppm of solvent is present in the fraction separated in step c).