HK1113803A - Process for the production of polyurethanes and/or prepolymers - Google Patents

Description

Process for producing polyurethane and/or prepolymer

The application is a divisional application of Chinese patent application with the application date of 2005, 2.4. 200510054188.9 entitled "method for producing very pure 2, 4' -methylene diphenyl diisocyanate".

Technical Field

The present invention relates to a process for the production of diisocyanates of the diphenylmethane series having a very high content of 2, 4' -methylenediphenyl diisocyanate, and to a process for the preparation of prepolymers and polymers from these diisocyanates.

Background

Aromatic isocyanates are important starting materials for the preparation of polyurethane materials. Among them, the diphenylmethane series of diisocyanates and polyisocyanates (MDI) are absolutely predominant in number.

Polyisocyanates of the diphenylmethane series are believed to be represented by the following types of isocyanates and isocyanate mixtures:

x is 2-n, where n represents an integer greater than 2.

Similarly, polyamines of the diphenylmethane series are believed to be represented by the following types of compounds and mixtures of compounds:

x is 2-n, where n represents an integer greater than 2.

It is known to prepare diisocyanates and polyisocyanates of the diphenylmethane series (MDI) by phosgenation of the corresponding diamines and polyamines of the diphenylmethane series (MDA). The di-and polyamines of the diphenylmethane series (MDA) themselves are prepared by condensation of aniline and formaldehyde. The corresponding diisocyanates 2, 2 ' -MDI, 2, 4 ' -MDI and 4, 4 ' -MDI, which are prepared by phosgenation of diamines of the diphenylmethane series, are described by the expert as 2-ring (i.e. binuclear) MDI compounds (i.e. diisocyanates of the diphenylmethane series). However, during the condensation of aniline and formaldehyde, the 2-ring (i.e. binuclear) MDA (methylene diphenyl diamine) continues to react further with formaldehyde and aniline to form higher core (i.e. polynuclear or polycyclic) types of MDA which, after phosgenation, constitute the polynuclear component in polymeric MDI (i.e. polyisocyanates of the diphenylmethane series).

The raw MDI produced in the phosgenation can be separated in a polymer/monomer separation by simple evaporation or distillation into 2-nuclear-MDI (i.e.monomeric MDI) and polymer-MDI fractions (i.e.polymeric MDI or PMDI). The 2-nuclear-MDI isomer mixture fraction contains, in addition to 2, 2 ' -MDI, 2, 4 ' -MDI and 4, 4 ' -MDI, secondary components such as solvent residues or phenyl isocyanate derivatives. According to the prior art, the monomeric 2-nuclear-MDI fraction is separated by distillation or crystallization into the 4, 4 ' -MDI isomers and a mixture comprising approximately 50% of 2, 4 ' -MDI and 50% of 4, 4 ' -MDI. These two monomeric products are then supplied to the world market as polyurethane raw materials or they are further processed with polymeric MDI into mixed products.

At present, very pure 2, 4' -MDI is not commercially available in large quantities. Although 2, 4' -MDI has recently been found to have a number of beneficial properties, the situation has not changed where it is not commercially available in large quantities. Thus, in polyurethane flexible foam systems, 2, 4' -MDI can replace the conventional TDI systems of 2, 4-TDI and 2, 6-TDI (as disclosed in EP-B10676434). 2, 4' -MDI can also be used successfully in heat-curable one-component polyurethane systems (as disclosed in EP-B10431331).

The 4, 4' -MDI isomer is the most important MDI isomer due to its high reactivity and excellent ability to form hard segments. By comparison, the 2, 4' isomers are particularly useful in prepolymers having very low viscosities and relatively low monomer content due to their different reactivity.

Thus, WO-A93/09158 discloses prepolymers with a low monomer content. By reacting isocyanates with polyethers containing secondary hydroxyl groups in a ratio NCO/OH of 1.6 to 1.8 and simultaneously using a plurality of reactive NCO groups in the isocyanate molecule, prepolymers with a low monomer content are produced. Monomeric MDI having a2, 4' -diphenylmethane diisocyanate content of 92% is given as an example of a suitable isocyanate. Important fields of application are adhesives and coatings, and in particular low-permeability composite film systems.

One application of diphenylmethane diisocyanate is in the preparation of composite films for the food packaging sector. Nowadays, inexpensive food packaging ensuring a satisfactory hygiene, high storage stability can be obtained by means of composite films. These composite films are formed by bonding films having different barrier properties so that the composite can be optimally tailored to various needs.

Polyurethane represents the adhesive of choice. These binders are used in solvent-containing or solvent-free form. Due to the trend towards solvent-free systems, two-component systems consisting of polyol mixtures and isocyanate-based prepolymers are becoming more popular. Due to the traces of moisture adsorbed on the film surface, it has proven necessary to use binders having a relatively large excess of isocyanate groups relative to OH groups.

However, since it must be ensured that the film composite is free of aromatic amines before packaging with the food, the above-mentioned large excess of isocyanates also becomes a limiting factor for further processing of the film composite.

Thus, the composite membrane must be stored until the amine is no longer detectable before it can be further processed. This period of time depends on many factors such as the nature of the adhesive applied, the properties of the film (such as type and thickness), and the prevailing temperature and air humidity. In this regard, the presence of aromatic amines in the test food may be due to the fact that the incompletely reacted monomeric isocyanates migrate through the film and slowly react with moisture at the surface to form polyureas, which are stable under normal food storage conditions. Until the reaction was complete, any monomeric isocyanate present was also partially hydrolyzed to amine by the food used in the test.

Since the NCO groups in the 4-position are much more reactive than the NCO groups in the 2-position in MDI, MDI isomers having at least one NCO group in the 4-position can be incorporated into polyurethane networks of oligomeric or polymeric nature considerably faster and thus migration through the film is avoided. Thus, the monomeric MDI isomers having NCO groups at the 4-position (i.e. 4, 4 '-MDI and 2, 4' -MDI), which are still free and unreacted, are rapidly reduced after the preparation of the MDI prepolymer, and also during the treatment of the MDI prepolymer with the polyol. On the other hand, 2' -MDI, due to its significantly lower reactivity, remains as a monomer in the adhesive layer for a longer time than the other monomers and thus takes longer to migrate through the film. The content of 2, 2' -MDI in the MDI isomer mixture is therefore a crucial quantity and should be as small as possible.

The following information is also available from the prior art regarding the preparation of MDI.

The production of MDI mixed products comprising various MDI isomers by specific synthesis of MDA comprising the corresponding MDA isomers is known and disclosed in the literature. EP-B1158059 discloses the preparation of specific high 2-core MDA mixtures comprising about 80% 4, 4 '-MDA and about 10% 2, 4' -MDA, the 2-core content being about 90%. On the other hand, the yield of 4, 4 '-MDA can be increased in a targeted manner, as disclosed in EP B13303, MDA comprising 88% by weight of 2-nuclear MDA, of which 19% by weight of 2, 2' -MDA, 36% by weight of 2, 4 '-MDA and 45% by weight of 4, 4' -MDA are present. Particularly, MDA rich in the monomer type contains a high content of 2, 4 '-MDA, and its production usually generates a large amount of 2, 2' -MDA as a byproduct. However, due to its lack of reactivity, the 2, 2 '-MDI formed from the 2, 2' -MDA produced is undesirable in high concentrations in many applications.

The preparation of MDA with a high polymer content having a 2-core content of 46% to 65% is disclosed, for example, in DE-A12750975 and DE-A12517301.

The necessary parameters for adjusting the proportion of 2, 4' -MDA in the polycondensation of aniline and formaldehyde are well known. Typically, the 2-nuclear MDA content is adjusted by the excess aniline in the polycondensation. In the polycondensation, the proportion of 2-nuclear 2, 4' -MDA present in 2-nuclear MDA is adjusted by the low degree of protonation, or, in other words, by hydrochloric acid: low molar ratios of aniline, such as < 0.2: 1, or by high reaction temperatures, as disclosed in DE-A13407497.

The preparation of isocyanates by reaction of the corresponding amines with phosgene in solvents is well known and is disclosed in detail in the literature (Ullmanns Enzyklopadie der technischen Chemie, 4 th edition, Vol.13, pp.347-357, Verlag Chemie GmbH, Weinheim, 1977). The MDA phosgenation first produces an untreated MDI mixture. In addition, the separation of monomeric and polymeric MDI from untreated MDI mixtures by distillation or crystallization has been substantially disclosed in the relevant literature.

Basically, the untreated 2-nuclear MDI fraction from the initially untreated MDI mixture is separated into two main products according to the prior art. The first of the two main products in the isomer separation is a mixture rich in 4, 4 '-MDA isomers ("4, 4' -product") which contains almost no 2, 2 '-MDI and which also contains < 3% by weight of 2, 4' -MDI. The second of the two main products in the isomer separation is a2, 4 '-MDA isomer-enriched mixture ("2, 4'/4, 4 '-product") comprising 20 to 70 wt% of 2, 4' -MDI and up to 3 wt% of 2, 2 '-MDI, the remainder being 4, 4' -MDI. To prepare these two main products, the following two industrial processes starting from untreated monomeric 2-nuclear MDI fractions are generally used today, the 2-nuclear MDI fractions being obtained from an untreated MDI mixture separated from the polymer/monomer:

a) distillation, as disclosed in DE-A13145010 and/or DE-A12631168;

or

b) Crystallization, as disclosed in EP-A2482490 and/or DE-A2532722.

Experts in the field of polyurethanes and polyisocyanates are currently concerned with the most economical preparation of isomeric mixtures of the "4, 4 ' -product" and the "2, 4 '/4, 4 ' -product" of monomeric nature (M. Stepanski, P. Faessler: "New hybrid Process for purification and separation of MDI isomers", Sulzer Chemtech, Presentation at the Polyurethane Conference 2002m Salt Lake City, 10/2002).

Processes for preparing higher concentrations of 2, 4 '-MDI starting from high contents of 2, 4' -MDA obtained by condensation of aniline and formaldehyde with a low degree of protonation of the aniline are disclosed, for example, in WO-A1

02/070581, respectively. No mention is made in the process described in WO-A102/070581 of purification involving removal of 2, 2 '-MDI from the 2, 4' -MDI obtained. However, especially in the preparation of 2, 4 '-MDA-rich mixtures with low protonation (i.e.with low hydrochloric acid to aniline ratios), disproportionately large amounts of 2, 2' -MDA are formed. This is disclosed, for example, in EP-B13303. These large amounts of 2, 2 '-MDA or 2, 2' -MDI, respectively, need to be at least partially removed after phosgenation or before the mixture is used in polyurethane production. In practice, however, the purity of 2, 4 ' -MDI is an indispensable quality feature with respect to the amount of 2, 2 ' -MDI present, which is much more important than the purity of 4, 4 ' -MDI.

In the process disclosed in WO-A102/070581, an MDI mixture having a high content of 2, 4 '-MDI is obtained by phosgenating a corresponding MDA mixture having a high content of 2, 4' -MDA, which corresponds to the conventional process disclosed in the prior art for preparing an MDI mixture comprising about 50% by weight of 2, 4 '-MDI and about 50% by weight of 4, 4' -MDI. Since 2, 4 ' -MDI is a low boiling compound relative to 4, 4 ' -MDI, 2, 4 ' -MDI is obtained as an overhead product by distillation. An important factor in this case is that further low boilers, in particular 2, 2' -MDI, accumulate in the overhead product. If 4, 4 '-MDI is separated from the 2, 4' -fraction by conventional methods, the first main product "4, 4 '-MDI product" already mentioned above, which generally comprises about 1 to 2% by weight of 2, 4' -MDI, is obtained here as the first product, and the second main product "2, 4 '/4, 4' -MDI product" also already mentioned above, forms a mixture of 2, 4 '-MDI and 4, 4' -MDI in the vicinity of the eutectic point, as the second product. Among them, the 2, 2' -isomer is aggregated in the eutectic mixture according to its boiling point. The typical content of 2, 2' -MDI in the eutectic mixture is from 0.8 to 5% by weight.

Similar conditions exist in the isolation of the "4, 4' -MDI product" by crystallization. In this case, the 2, 2 '-isomer must be concentrated with the 2, 4' -enriched fraction in the mother liquor. If the mother liquor formed is then separated into the 2, 4 '-MDI isomer as a distillate and 4, 4' -MDI at the bottom, the undesired and unreactive 2, 2 '-MDI likewise accumulates in the desired 2, 4' -MDI fraction. Thus, 2, 4 '-MDI was obtained containing 0.8 to 5% by weight of 2, 2' -MDI, based on the initial quality of the untreated 2-nuclear MDI fraction. Further, if it is not initially removed from the starting mixture, low-boiling secondary components such as phenyl isocyanate and traces of solvent may enter the distillate.

On the other hand, if it is attempted to separate the resulting mother liquor comprising 2, 4 ' -MDI and 4, 4 ' -MDI isomers by crystallization, it is first necessary to increase the eutectic point using a non-crystallizing process in order to avoid obtaining only pure 4, 4 ' -MDI crystals and a2, 4 ' -containing mother liquor with a high content of 4, 4 ' -MDI. Thus, non-highly concentrated 2, 4' -MDI can be obtained from a pure crystallization process starting from untreated 2-nuclear MDI fractions. The eutectic point may be increased by distillation such as described above. In this case the process disclosed in the present application for preparing a very pure 2, 4 '-MDI fraction can be carried out in step d) using additionally a crystallization process for separating off the major portion of 4, 4' -MDI.

In DE-A2631168, a process for preparing MDI mixtures having a low residual chlorine content by distillation is disclosed. MDI mixtures having a2, 4' -MDI content of more than 97% by weight and a chlorine content of less than 50ppm are also prepared in this way by the process disclosed in DE-A2631168.

Since 2, 2 '-MDI is non-reactive and only under severe reaction conditions is completely incorporated into the polymer network, high levels of 2, 2' -MDI interfere in practical end-use applications. In most cases, large amounts of 2, 2' -MDI are present as residual monomers in the processing and may be released over time or react with atmospheric moisture in any way impairing the properties of the polymer. Traces of residual solvent are also undesirable in polyurethane preparation, since solvents have an unpleasant odor, which adversely affects product quality. Traces of phenyl isocyanate which may be present in turn may act as chain terminators for the polyurethane reaction and also adversely affect the polymer properties.

Disclosure of Invention

The object of the present invention is to provide a process for the production of MDI isomer mixtures having a high 2, 4 '-MDI content, while reducing the content of components such as 2, 2' -MDI, residual solvents and phenyl isocyanate to a level which no longer affects the preparation of the polyurethane.

The present invention relates to a process for the preparation of a fraction of diisocyanates of the diphenylmethane series comprising at least 99% by weight, based on the total weight of the fraction, of 2-nuclear methylene diphenyl diisocyanate. The method comprises the following steps:

a) reacting aniline and formaldehyde in the presence of an acid catalyst to form diphenylmethanediamines and polyamines comprising 2-nuclear methylenediphenyldiamine,

b) phosgenating di-and polyamines of the diphenylmethane series comprising 2-nuclear methylenediphenyl diamine, optionally in the presence of a solvent, thereby forming untreated di-and polyisocyanates,

c) separating a fraction comprising at least 95% by weight of 2-nuclear methylene diphenyl diisocyanate, based on the total weight of the fraction, of the untreated diisocyanates and polyisocyanates formed in step b), wherein the 4, 4 ' -MDI content is from 49 to 95.99% by weight, the 2, 4 ' -MDI content is from 4 to 45% by weight and the 2, 2 ' -MDI content is from 0.01 to 20% by weight,

d) optionally, 10 to 98% of the 4, 4' -MDI is removed from the fraction obtained in step c),

e) separating 2, 2 '-MDI completely or partially from the fraction obtained in step c) or step d), thereby forming a fraction comprising 0 to 0.4% by weight of 2, 2' -MDI, 1 to 95% by weight of 4, 4 '-MDI and 5 to 98.6% by weight of 2, 4' -MDI, based on the total weight of MDI isomers,

and in preferred embodiments optionally

f) Separating a fraction comprising at least 99% by weight, based on the total weight of the fraction, of 2-nuclear methylene diphenyl diisocyanate from the fraction comprising 0 to 0.4% by weight of 2, 2 '-MDI, 1 to 95% by weight of 4, 4' -MDI and 5 to 98.6% by weight of 2, 4 '-MDI, based on the total weight of the MDI isomers, of 2-nuclear methylene diphenyl diisocyanate, based on the total weight of the MDI isomers, of 0 to 0.5% by weight of 2, 2' -MDI, 0.1 to 80% by weight of 4, 4 '-MDI and 20 to 99.9% by weight of 2, 4' -MDI.

The essence of the invention is a process aimed at removing 2, 2' -MDI and residual solvents and phenyl isocyanates from isomeric mixtures of monomeric nature by separation processes, especially distillation. Since phenyl isocyanates, solvents such as monochlorobenzene and o-dichlorobenzene, and 2, 2 ' -MDI isomers have lower boiling points than 4, 4 ' -MDI and 2, 4 ' -MDI, they accumulate steadily in the fraction from the distillation column. However, in principle, crystallization or extraction can also be used. The MDI mixtures prepared and purified according to the invention with a high content of 2, 4' -MDI are used in polyurethane production with extremely high results.

Detailed Description

Polyamines of the diphenylmethane series or polyamine mixtures used in the process according to the invention are produced in step a) by condensation of aniline and formaldehyde in the presence of an acid catalyst.This is well known and described in, for example, h.j.twitchett, chem.soc.rev.3(2), 209 (1974); moore in, w.m.: chem.technol.3. Kirk-Othmer encycle^thEd., New York, 2, 339-. For the process of the present invention, it is not important whether the aniline and formaldehyde are first mixed in the absence of an acid catalyst and then an acid catalyst is added, or whether a mixture of aniline and acid catalyst reacts with formaldehyde.

Suitable polyamine mixtures of the diphenylmethane series are generally obtained by polycondensation of aniline and formaldehyde in a molar quantity ratio of from 20: 1 to 1.6: 1, preferably from 10: 1 to 1.8: 1, and aniline and acid catalyst in a molar quantity ratio of from 20: 1 to 1: 1, preferably from 10: 1 to 2: 1.

Formaldehyde is generally used in the industrial field as an aqueous solution. In this case, the water content is 1 to 95 wt% based on the total weight of the solution. Preferably, the aqueous solution used comprises 50 to 80 wt% of water (based on the total weight of the solution). However, other methylene-donating compounds, such as polyoxymethylene glycol, paraformaldehyde or trioxane, may also be used.

Strong organic acids, and preferably inorganic acids, have proven suitable as acid catalysts for the reaction of aniline and formaldehyde. Suitable acids include, for example, hydrochloric acid, sulfuric acid, phosphoric acid and methanesulfonic acid. Hydrochloric acid is preferably used. However, solid acid catalysts, such as organic and inorganic ion exchangers, acidic silicon/aluminum mixed oxides and preferably acidic zeolites, can also be used.

In a preferred embodiment of the process, aniline and acid catalyst are first mixed together. Optionally after removal of heat, the mixture is mixed in a further step with formaldehyde at about 20 ℃ to 100 ℃, preferably about 30 ℃ to 70 ℃ in a suitable manner and then subjected to a preliminary reaction in an apparatus with a suitable residence time. The preliminary reaction is carried out at a temperature of about 20 ℃ to 100 ℃, preferably about 30 ℃ to about 80 ℃. Once mixing and preliminary reaction have been completed, the temperature of the reaction mixture is increased either in stages or continuously, and optionally under overpressure, to a temperature of from about 100 ℃ to about 250 ℃, preferably from about 100 ℃ to about 180 ℃, particularly preferably from about 100 ℃ to about 160 ℃.

However, in another embodiment, aniline and formaldehyde may be first mixed and reacted at a temperature of from about 5 ℃ to about 130 ℃, preferably from about 40 ℃ to about 100 ℃, particularly preferably from about 60 ℃ to about 90 ℃, in the absence of an acid catalyst. In this way, condensation products of aniline and formaldehyde (so-called aminals) are formed. Once the aminal formation is complete, the water present in the reaction mixture may be removed by phase separation or other suitable process, such as distillation. The polycondensation product is then mixed in a suitable manner with the acid catalyst in a further process step, which is then subjected to a preliminary reaction at from about 20 ℃ to about 100 ℃, preferably from about 30 ℃ to about 80 ℃, in an apparatus having a residence time. The reaction mixture is then brought to a temperature of from about 100 ℃ to about 250 ℃, preferably to a temperature of from about 100 ℃ to about 180 ℃, particularly preferably to a temperature of from about 100 ℃ to about 160 ℃, optionally under overpressure, in stages or continuously.

To work up the acid reaction mixture, the reaction mixture is neutralized with a base as disclosed in the prior art. Neutralization is generally carried out at temperatures of from 90 ℃ to 100 ℃ according to the prior art (see H.J.Twitchett, chem.Soc.Rev.3(2), 223 (1974)). Suitable bases include, for example, the hydroxides of alkali metals and alkaline earth metals. NaOH (sodium hydroxide) is preferably used for neutralizing the reaction mixture.

After neutralization, the organic phase is separated from the aqueous phase by suitable methods, for example by phase separation in a separating flask, as described in the prior art. The separation of the organic phase from the aqueous phase can be accomplished at the same temperature as the temperature of the neutralization reaction of the acid rearrangement mixture. The organic phase containing the product, which remains after the separation of the aqueous phase, is subjected to a washing process to separate out the salts and excess base. This washing process is also disclosed in the prior art. The purified organic phase is then freed of excess aniline and other substances present in the mixture (e.g. further solvents) by suitable physical separation methods, such as distillation, extraction and crystallization.

The diphenyl obtained in step a)Polyamines of the methyl-ene series (untreated MDA) are reacted in step b) with phosgene in a known manner, optionally in an inert organic solvent, to give the corresponding isocyanates. The molar ratio of untreated MDA relative to phosgene is expediently adjusted to 1 mol of NH per mole of NH present in the reaction mixture₂The radicals, in the reaction mixture, are present in the amount of 1 to 10 mol, preferably 1.3 to 4 mol, of phosgene. Suitable inert solvents include chlorinated aromatic hydrocarbons, such as monochlorobenzene, dichlorobenzene, trichlorobenzene, the corresponding toluenes and xylenes, and also chlorinated ethylbenzene. In particular monochlorobenzene, bischlorobenzene or mixtures of these chlorobenzenes can be used as inert organic solvents. The amount of solvent is preferably adjusted to the extent that the reaction mixture has from 2 to 40% by weight, preferably from 5 to 20% by weight, of isocyanate, based on the total weight of the reaction mixture. Here, the untreated MDA is reacted with phosgene at temperatures of from 50 to 250 ℃ and pressures in the range from atmospheric pressure to 50 bar. The reaction is preferably carried out at a temperature of from 70 to 180 ℃ and at a pressure of from 2 to 20 bar.

After the phosgenation step is complete, the excess phosgene, any inert organic solvent, the HCl formed and/or mixtures thereof are separated from the reaction mixture by suitable methods, for example by distillation. For this purpose, the pressure was gradually reduced to generate a vacuum and the excess phosgene remaining and the HCl formed were separated off by evaporation. The solvent is then reduced stepwise, preferably by evaporation, with further reduction of the absolute pressure to 1mbar, preferably to 5 mbar. At the same time, the temperature was raised until the solvent was almost completely removed, reducing to a concentration well below 0.1%. Finally, untreated diisocyanate and polyisocyanate (untreated MDI mixture) are obtained in this step b).

Then in a polymer/monomer separation in step c) a fraction is separated from the untreated diisocyanate and polyisocyanate, which fraction comprises at least 95% by weight of 2-nuclear methylene diphenyl diisocyanate, based on the total weight of the fraction, with a 4, 4 ' -MDI content of 49 to 95.99% by weight, a2, 4 ' -MDI content of 4 to 45% by weight and a2, 2 ' -MDI content of 0.01 to 20% by weight. The separation of the 2-nuclear MDI fraction (i.e.monomeric MDI) is preferably carried out in one or two stages. The low-boiling fraction having a high 2-nuclear MDI isomer content is preferably evaporated overhead and removed at a temperature of 170-230 ℃ and at an absolute pressure of 0.3 to 30 mbar. Preferably the temperature is 180 ℃ and 220 ℃ and the absolute pressure is 1-15 mbar.

Preferably, the fraction separated in step c) comprises 98 to 100% by weight of 2-nuclear methylene diphenyl diisocyanate, based on the total weight of the fraction, wherein the 4, 4 ' -MDI content is 80 to 93.9% by weight, the 2, 4 ' -MDI content is 6 to 19.9% by weight and the 2, 2 ' -MDI content is 0.1 to 5% by weight, based on the total weight of the fraction. Particularly preferably, the fraction separated in step c) comprises at least 99% by weight, based on the total weight of the fraction, of 2-nuclear methylene diphenyl diisocyanate having a 4, 4 ' -MDI content of 82 to 92.8% by weight, a2, 4 ' -MDI content of 7 to 17.8% by weight and a2, 2 ' -MDI content of 0.2 to 3% by weight.

The 4, 4' -MDI isomer is the most commonly used MDI isomer in polyurethane preparation. It may therefore be appropriate to first separate off the major part of the 4, 4' -MDI from the fraction obtained in step c). Preferably, therefore, from 10 to 98% by weight, more preferably from 50 to 95% by weight, most preferably from 75 to 93% by weight of 4, 4' -MDI are separated off from the distillation stage or the crystallization stage in step d). However, in step d), the separation or removal of 4, 4' -MDI from the fraction obtained in step c) is optional. The distillation column used in step d) is preferably operated at from 170 ℃ to 230 ℃ and at an absolute pressure of from 0.1 to 30 mbar. It is particularly preferred that the 4, 4' -MDI-rich stream is withdrawn from the bottom of the distillation column with a temperature adjustment of from 180 ℃ to 220 ℃ and an absolute pressure adjustment of from 1 to 15 mbar. Step d) may be additionally carried out by one or more crystallization steps at a temperature of from 30 ℃ to 40 ℃ with the 4, 4 ' -enriched fraction present as crystals and the 2, 4 ' -enriched fraction and the 2, 2 ' -enriched fraction as mother liquor. Thus, preferably from 10 to 98% by weight, more preferably from 50 to 95% by weight and most preferably from 75 to 93% by weight of 4, 4' -MDI are separated off as crystals during the crystallization in step d).

Then in step e) 2, 2 '-MDI is completely or partially separated from the fraction obtained in step c), which fraction comprises at least 95% by weight of 2-endomethylenediphenyl diisocyanate, wherein the content of 4, 4' -MDI is from 49 to 95.99% by weight, the content of 2, 4 '-MDI is from 4 to 45% by weight and the content of 2, 2' -MDI is from 0.01 to 20% by weight, based on the total weight of the fraction. In an optional step d), after a large amount of 4, 4 '-MDI has been separated off, the separation of 2, 2' -MDI is carried out in whole or in part.

The 2, 2 '-enriched fraction obtained as distillate in step e) comprises at least 99 wt% of 2-nuclear MDI and comprises 10-98 wt% of 2, 2' -MDI, 2-90 wt% of 2, 4 '-MDI and 0-30 wt% of 4, 4' -MDI, based on the total weight of the MDI isomers. The 2, 2 ' -MDI enriched fraction preferably comprises from 20 to 95 wt% of 2, 2 ' -MDI, from 5 to 80 wt% of 2, 4 ' -MDI and from 0 to 20 wt% of 4, 4 ' -MDI, based on the total weight of the MDI isomers, and more preferably from 50 to 85 wt% of 2, 2 ' -MDI, from 15 to 50 wt% of 2, 4 ' -MDI and from 0 to 10 wt% of 4, 4 ' -MDI. Thus, from 50 to 99.9999 wt.% (i.e. 100%), more preferably from 65 to 99.99 wt.% and most preferably from 80 to 99.9 wt.% of the total amount of 2, 2' -MDI introduced with the untreated diisocyanate and polyisocyanate in the separation (e.g. distillation) in step c) is removed as a distillation stream.

The 2, 2 '-MDI separated in step e) is then separated in a further step by isomer distillation into a low-boiling fraction and a2, 2' -MDI-rich fraction. Interfering low-boiling compounds, such as derivatives of phenylisocyanates, solvent components and other readily volatile secondary products of the MDA and MDI production, such as acridine, are removed overhead as distillate. For this purpose, a portion of the MDI, for example from 5 to 30% of the feed, preferably from 10 to 20% of the feed, is used as entrainer in the distillation stream. The resulting higher boiling fraction contains a large portion of the useful MDI product. Similarly, in one or more crystallization steps, interfering low boilers can be separated as mother liquor from the MDI useful product.

Thus, MDI isomer mixtures having a high content of 2, 2 '-MDI, wherein the 2, 2' -MDI content is from 5 to 99.99% by weight, preferably from 10 to 95% by weight, more preferably from 20 to 90% by weight and most preferably from 30 to 85% by weight, based on the total weight of the MDI isomers, can be obtained by distillative separation of interfering low boiling compounds. The above-described isomeric mixture having a high 2, 2 ' -MDI content of from 5 to 99.99% by weight of 2, 2 ' -MDI, from 0 to 50% by weight of 4, 4 ' -MDI and from 0.01 to 95% by weight of 2, 4 ' -MDI, preferably from 10 to 95% by weight of 2, 2 ' -MDI, from 0.1 to 89.99% by weight of 2, 4 ' -MDI and from 0.01 to 50% by weight of 4, 4 ' -MDI, based on the weight of the MDI isomers, may then be reacted with a polyether or polyester to form a polyurethane or prepolymer in a well-known manner. Alternatively, these isomeric mixtures having a high 2, 2 ' -MDI content can also be used in combination with other isocyanate products, including other MDI mixed products, for the supplementation of the 2, 2 ' -content and the 2, 4 ' -content and for the preparation of polyurethanes or prepolymers thereof with polyethers or polyesters.

The separation of 2, 2 '-MDI in step e) is preferably carried out in the stripping section of the column in a distillation column containing at least 10, preferably at least 15 and most preferably at least 20 theoretical plates in order to minimize the 2, 2' -MDI content in the bottom of the column. For this purpose, the reflux ratio (i.e.the amount of distillate refluxed/the amount of distillate removed in the column) is adjusted at the top of the column to be from 0.5 to 500. The reflux ratio is preferably adjusted to 2 to 100. The column is operated at an absolute pressure of from 0.5 to 30mbar, preferably from 1 to 15 mbar.

Alternatively, a column which removes a fraction with a low 2, 2' -MDI content in a side stream may also be used. In this case, the stripping section of the column between the feed opening for the stream of step c) or step d) and the discharge opening for the low 2, 2' -MDI-content product must have a separation efficiency of at least 10, preferably at least 15 and more preferably at least 20 theoretical plates. The fraction with a low 2, 2' -MDI content is removed here from the bottom of the column in gaseous or liquid form. For this purpose, the reflux ratio at the top of the column (i.e. the amount of distillate refluxed/the amount of distillate removed in the column) is adjusted to 0.5 to 500. Preference is given to using a reflux ratio of from 2 to 100. The column is operated at an absolute pressure of from 0.5 to 30mbar, preferably from 1 to 15 mbar.

After separation of the 2, 2 '-MDI, a fraction comprising from 0 to 0.4% by weight of 2, 2' -MDI, from 1 to 95% by weight of 4, 4 '-MDI and from 5 to 98.6% by weight of 2, 4' -MDI, based on the total weight of the MDI isomers, is obtained in step e).

The fraction obtained in step e) preferably comprises a fraction of 0 to 0.3 wt.% 2, 2 '-MDI, 5 to 85 wt.% 4, 4' -MDI and 15 to 95 wt.% 2, 4 '-MDI, based on the total weight of the MDI isomers, more preferably 0 to 0.18 wt.% 2, 2' -MDI, 7 to 75 wt.% 4, 4 '-MDI and 25 to 93 wt.% 2, 4' -MDI. Most preferably the fraction obtained comprises a fraction of 0-0.10 wt.% 2, 2 ' -MDI, 30-70 wt.% 4, 4 ' -MDI and 30-70 wt.% 2, 4 ' -MDI, based on the total weight of the MDI isomers. The desired 2, 4 '-enriched fraction and the 2, 2' -reduced fraction can optionally be removed first in this step by means of partial condensation of the vapors, for example above the bottom phase, whereby the separate distillation in step f) can be omitted. This is a preferred embodiment for preparing 2, 4 ' -MDI/4, 4 ' -MDI eutectic mixtures with 2, 2 ' -eliminated, in which the mixture comprises 0 to 0.40 wt.% of 2, 2 ' -MDI, 30 to 70 wt.% of 4, 4 ' -MDI, 30 to 70 wt.% of 2, 4 ' -MDI, based on the total weight of the MDI isomers, and preferably 0 to 0.20 wt.% of 2, 2 ' -MDI, 35 to 60 wt.% of 4, 4 ' -MDI, 40 to 60 wt.% of 2, 4 ' -MDI, based on the total weight of the MDI isomers.

Optionally in a preferred embodiment, in step f) a fraction comprising at least 99 wt%, based on the total weight of the fraction, of 2-nuclear MDI (which fraction comprises 0 to 0.5 wt%, based on the total weight of the MDI isomers, of 2, 2 '-MDI, 0.1 to 80 wt% of 4, 4' -MDI and 20 to 99 wt% of 2, 4 '-MDI) is finally separated from the fraction obtained in step e), the fraction obtained in step e) comprising 0 to 0.4 wt%, based on the total weight of the MDI isomers, of 2, 2' -MDI, 1 to 95 wt% of 4, 4 '-MDI and 5 to 98.6 wt% of 2, 4' -MDI. The distillation is preferably carried out at an absolute pressure of from 0.5 to 30mbar, preferably from 1 to 15 mbar. A column comprising at least 1, preferably at least 5 and more preferably at least 10 theoretical plates is used for the distillation. Alternatively, if the fraction of step e) has a2, 4' -MDI content of more than 60% by weight based on the total weight of MDI isomers, it can also be purified by crystallization. In this case, by partial crystallization, a crystalline phase with a high 2, 4 '-MDI content is obtained and a mother liquor with a high 4, 4' -MDI content is obtained.

In step f), various 2-nuclear MDI mixtures having different contents of 2, 4 ' -MDI and 4, 4 ' -MDI are obtained by distillation and the 2, 2 ' -MDI content in the mixture is very low. If desired, moderately concentrated or highly concentrated 2, 4' -MDI products can be obtained batchwise in a distillation column. Alternatively, 2 or more distillation columns can be operated in series one after the other, whereby various 2-nuclear MDI mixtures can be prepared simultaneously.

Preferably, the fraction obtained in step f) comprises 0 to 0.35 wt.% of 2, 2 ' -MDI, 0.2 to 60 wt.% of 4, 4 ' -MDI and 40 to 99.8 wt.% of 2, 4 ' -MDI, based on the total weight of the MDI isomers. More preferably, a fraction is produced comprising 0 to 0.2 wt.% 2, 2 ' -MDI, 0.5 to 55 wt.% 4, 4 ' -MDI and 45 to 99.5 wt.% 2, 4 ' -MDI, based on the total weight of the MDI isomers. In step f), it is most preferred that the resulting fraction comprises 0 to 0.2 wt.% of 2, 2 '-MDI, 0.5 to 10 wt.% of 4, 4' -MDI and 90 to 99.5 wt.% of 2, 4 '-MDI, based on the total weight of the MDI isomers, or that the resulting fraction comprises 0 to 0.2 wt.% of 2, 2' -MDI, 35 to 60 wt.% of 4, 4 '-MDI and 40 to 65 wt.% of 2, 4' -MDI, based on the total weight of the MDI isomers. Thus, the MDI purity in the fractions amounts to > 99.9 wt.%.

The solvent content in the fraction obtained in step f) is preferably from 0 to 5ppm, more preferably from 0 to 1ppm, and most preferably from 0 to 0.5ppm, based on the total weight of the MDI isomers. The content of phenyl isocyanate in the fraction obtained in step f) is preferably from 0 to 5ppm, more preferably from 0 to 1ppm, and most preferably from 0 to 0.5ppm, based on the total weight of the MDI isomers.

The invention also relates to a process for the production of very pure 2, 4 ' -MDI, wherein fraction e) or fraction f) comprising 30 to 70 wt.% 4, 4 ' -MDI is concentrated in the further isomeric fraction to very pure 2, 4 ' -MDI comprising 0 to 10 wt.% 4, 4 ' -MDI, preferably 0 to 5 wt.% 4, 4 ' -MDI, more preferably 0 to 3 wt.% 4, 4 ' -MDI, and comprising 0 to 1 wt.% 2, 2 ' -MDI, preferably 0 to 0.5 wt.% 2, 2 ' -MDI, and more preferably 0 to 0.2 wt.% 2, 2 ' -MDI.

The invention also relates to a process in which, for example, step d) and step e), or step e) and step f), and low boilers are separated off from the 2, 2' -MDI fraction separated off in step e), which is carried out in a sidestream distillation column with three draw-off streams, and finally a product of the same composition is obtained. In this case, some of the intermediates are not isolated one by one at all.

The invention also relates to a process for preparing prepolymers and polyurethanes, wherein pure 2, 4 '-MDI or the fraction prepared in step e) or the fraction prepared in step f), which comprises 0 to 0.5% by weight of 2, 2' -MDI, 0.1 to 80% by weight of 4, 4 '-MDI and 20 to 99.9% by weight of 2, 4' -MDI, based on the total weight of the MDI isomers, is reacted with a polyether or polyester.

All ranges used in this application include upper and lower limits unless otherwise stated. All ranges given also use any combination of upper and lower limits inclusive, unless otherwise specified.

The following examples further illustrate the process of the present invention in detail. The invention, as set forth in the foregoing disclosure, is not intended to be limited in spirit or scope by these examples. Those skilled in the art will readily appreciate that the conditions of the following processes may be varied as is well known. Unless otherwise indicated, all temperatures are degrees Celsius and all percentages are percentages by weight.

Examples

The process according to the invention is illustrated below by means of examples, in which all numerical% denote weight% (wt%).

To prepare the isomer mixture comprising 2, 2 '-MDI and comprising 2, 4' -MDI, the MDA base (i.e.di-or polyamines of the diphenylmethane series) is first prepared conventionally from aniline and formaldehyde. The MDA base is then phosgenated and separated by distillation into a monomer fraction and a polymer fraction. The monomer fraction thus obtained (i.e.2-nuclear MDI) is separated into isomers by distillation. The specific analytical results of the MDI isomers together with the secondary products monochlorobenzene and phenyl isocyanate were determined by gas chromatography. The HC component of hydrolyzable chlorine is determined by titration.

Example 1(not in accordance with the invention)

In step a), 1000 g of aniline are mixed with 306 g of 31.9% aqueous hydrochloric acid in a stirred tank at 40 ℃. 480 g of a 32% aqueous formaldehyde solution were added dropwise thereto over 15 minutes. The mixture was first stirred at 40 ℃ for a further 15 minutes, after which the temperature was slowly raised to 100 ℃ over the next 2.5 hours. The reaction mixture was then stirred at 100 ℃ for 10 hours under reflux, then neutralized with 50% sodium hydroxide solution, the aqueous phase was separated and the organic phase was washed with water. The organic solution was removed by vacuum distillation and the excess aniline was removed.

In step b), the MDA reaction product is poured into an ice-cold 15% phosgene solution in Monochlorobenzene (MCB) which has been placed beforehand in a second stirred tank; the molar excess of phosgene was 200%. With continuous addition of 40 l/h phosgene, the reaction solution was slowly heated to 100 ℃ over 1 hour. The mixture reached boiling point in the next hour, phosgene addition was stopped, and then a vacuum was applied. The temperature was increased stepwise to 210 ℃ and the absolute pressure was reduced to 3mbar, whereby the solvent was completely removed. An untreated MDI mixture was formed. The untreated MDI mixture contained 58 wt% MDI monomer based on the total weight of MDI isomers and oligomers.

The untreated MDI obtained in step b) is then separated via step c) into the polymeric MDI product, and the monomer fraction (i.e. 2-nuclear MDI). The test apparatus consists of a glass container with a droplet separator installed in the vapor space. The distillate is discharged via the top of the column and completely condensed and removed. The pressure was adjusted to 5mbar absolute. The untreated MDI was fed to the continuously operated apparatus at 180 ℃. The composition of the following products was determined under steady state equilibrium conditions:

bottom: polymeric MDI mixtures with a viscosity of 185mPas at 25 DEG C

Distillation unit overhead stream [ ═ fraction c ]:

comprising 0.54% of 2, 2 ' -MDI, 11.29% of 2, 4 ' -MDI and 88.17% of 4, 4 ' -MDI, based on the total weight of the MDI isomers.

The above fraction c constitutes the starting mixture for the separation of the isomers described below.

Fraction c) is fed in step d) to a continuously operated laboratory packed column having 10 theoretical plates in both the stripping section and the rising section (isomer distillation). The pressure at the top of the column was adjusted to 3 mbar. The feed temperature was 175 ℃. The overhead distillate was condensed by a condenser at 100 ℃. The condensed product is partially recycled and distilled via a splitter, and the remainder is removed as a distillation fraction. The reflux ratio (reflux/distillate) was 7 and 20% of the feed was distillate and 80% was bottoms, the resulting bottoms composition comprised 0% 2, 2 ' -MDI, 1.4% 2, 4 ' -MDI and 98.6% 4, 4 ' -MDI. Fraction d) had a distillation mass of 2.2% of 2, 2 ' -MDI, 41.9% of 2, 4 ' -MDI and 55.9% of 4, 4 ' -MDI. The above percentages are based on the total weight of the MDI isomers.

The distillation fraction d) is passed into a further glass column for distillation. The glass column contained 10 theoretical plates in both the stripping section and the enrichment section. The fraction d) is metered continuously into this stage e) at 175 ℃ between the stripping section and the enrichment section. At 3mbar absolute, 6% of the feed was removed from the top of the column at a reflux ratio of 12. The resulting distillate phase contained 18.4% 2, 2 ' -MDI, 70.6% 2, 4 ' -MDI and 11.0% 4, 4 ' -MDI, based on the total weight of the MDI isomers. The distillation phase also contained 0.3% of organic compounds derived from the secondary products formed in the MDI production. The bottom fraction e) had the following composition: based on the total weight of the MDI isomers, 1.2% 2, 2 ' -MDI, 40.1% 2, 4 ' -MDI and 58.7% 4, 4 ' -MDI.

In the final step f) the fraction e) is highly purified by distillation and separated in a further glass column having 10 theoretical plates in both the stripping section and the enrichment section. Fraction e) is metered continuously at 175 ℃ into the region between the stripping section and the enrichment section. At 3mbar absolute, 2/3 feed was removed from the top of the column at a reflux ratio of 1. The resulting distillate phase contained 1.79% 2, 2 ' -MDI, 52.74% 2, 4 ' -MDI and 45.74% 4, 4 ' -MDI, based on the total weight of the MDI isomers, and also 1ppm monochlorobenzene, 1ppm phenyl isocyanate and 10ppm hydrolysable chlorine. The resulting bottom phase is combined with the untreated MDI mixture (i.e.fraction b) from example 1) and recycled to steps c) to f).

Example 2(according to the invention)

The method described above in example 1 was followed until step d) was completed. From this point on, embodiment 2 is described as follows differently from embodiment 1.

Fraction d) from example 1 is fed to step e) for further separation of isomers by distillation. The glass column contained 10 theoretical plates in the enrichment section and 20 theoretical plates in the stripping section (packing under the feed distributor). Fraction d) is metered continuously at 175 ℃ into the region between the stripping section and the enrichment section. At 3mbar absolute, 6% of the feed was recovered at reflux ratio 10 at the top of the column. The resulting distillate phase contained 36.8% 2, 2 ' -MDI, 62.4% 2, 4 ' -MDI and 0.8% 4, 4 ' -MDI, based on the total weight of the MDI isomers. The distillation phase additionally contained 0.3% of organic compounds derived from secondary products formed in the preparation of MDI. The bottom fraction e) had the following composition: based on the total weight of the MDI isomers, 0.05% 2, 2 ' -MDI, 40.65% 2, 4 ' -MDI and 59.3% 4, 4 ' -MDI.

In the final step f) the fraction e) is highly purified by distillation and separated in a further glass column having 10 theoretical plates in both the stripping section and the enrichment section. Fraction e) is metered continuously at 175 ℃ into the region between the stripping section and the enrichment section. The feed of 2/3 was recovered at a reflux ratio of 1 (distillation stream: reflux stream) overhead at 3mbar absolute. The distillation phase, fraction f), was obtained as the target product and contained 0.07%, 52.52%, 2, 4 '-MDI and 47.41%, 4' -MDI, based on the total weight of the MDI isomers, and also 0.2ppm of monochlorobenzene, 0.1ppm of phenyl isocyanate and 7ppm of hydrolysable chlorine. The resulting bottom phase is combined with the untreated MDI mixture, i.e.fraction b) from example 1, and recycled to steps c) to f).

Example 3(not according to the invention)

The fraction e) of example 1 is highly purified by distillation in step f) and separated in a further glass column having 10 theoretical plates in both the stripping section and the enrichment section. Fraction e) is metered continuously at 175 ℃ into the region between the stripping section and the enrichment section. At 3mbar absolute, 35% of the feed was withdrawn from the top of the column at a reflux ratio of 2 (distillation stream-reflux stream). The distillation phase, fraction f), was obtained as the target product, which contained 3.37% 2, 2 ' -MDI, 95.12% 2, 4 ' -MDI and 1.51% 4, 4 ' -MDI, based on the total weight of the MDI isomers. The resulting bottom phase is combined with the untreated MDI mixture, i.e.fraction b) from example 1, and recycled to steps c) to f).

Example 4(in accordance with the invention)

The fraction e) of example 2 is now highly purified by distillation in the final step f). For this purpose, a further glass column having 10 theoretical plates in both the stripping section and the enrichment section was used. Fraction e) is metered continuously at 175 ℃ into the region between the stripping section and the enrichment section. At 3mbar absolute, 30% of the feed was withdrawn from the top of the column at a reflux ratio of 2. The distillation phase, fraction f), was obtained as the target product, which contained 0.16% 2, 2 ' -MDI, 97.05% 2, 4 ' -MDI and 2.79% 4, 4 ' -MDI, based on the total weight of the MDI isomers. The resulting bottom phase is combined with the untreated MDI mixture, i.e.fraction b) from example 1, and recycled to steps c) to f).

Table 1: gas chromatography analysis data of the fractions prepared in step f) of examples 1-4.

Example No.	1	2	3	4
					2，2’-MDI	1.79	0.07	3.37	0.16
2，4’-MDI	52.74	52.52	95.12	97.05
					4，4’-MDI	45.47	47.41	1.51	2.79

Example 5(in accordance with the invention)

The final product of example 2, fraction f), is distilled in a further step to give very pure 2, 4' -MDI. For this purpose, glass columns are used, both the stripping section and the enrichment section having 10 theoretical plates. The fraction f) from example 2 was metered continuously at 175 ℃ into the region between the stripping section and the enrichment section. At 3mbar absolute, 50% of the feed was withdrawn from the top of the column at a reflux ratio of 2. A distillation phase, fraction f), comprising 0.15% 2, 2 ' -MDI, 96.86% 2, 4 ' -MDI and 2.99% 4, 4 ' -MDI, based on the total weight of the MDI isomers, was obtained as the target product. The resulting bottom phase is combined with the untreated MDI mixture, i.e.fraction b) from example 1, and recycled to steps c) to f).

Example 6(in accordance with the invention)

The distillation fraction e) of example 2 is purified by distillation in a further step to give the usable MDI fraction g). For this purpose, glass columns are used, both the stripping section and the enrichment section having 10 theoretical plates. Example 2 the fraction of step e) was metered continuously at 175 ℃ into the region between the stripping section and the enrichment section. At 3mbar absolute, 20% of the feed was withdrawn from the top of the column at a reflux ratio of 5. The bottom phase, fraction g), was obtained as the target product, which contained 28.2% 2, 2 ' -MDI, 70.9% 2, 4 ' -MDI and 0.9% 4, 4 ' -MDI, based on the total weight of the MDI isomers. The resulting distillate phase, in addition to the main component 2, 2' -MDI, contains a number of low-boiling secondary components of the MDI production and is fed to a heat utilization unit.

The above examples demonstrate that by using a very large number of stripping sections in the separation column of step e), the concentration of 2, 2' -MDI in the MDI isomer mixture can be minimized. The product obtained from the isomer separation column modified in this way can be used for the preparation of MDI isomer mixtures which are virtually free of the 2, 2 '-isomer, for example for the preparation of very pure 2, 4' -MDI or for the preparation of mixtures of 2, 4 '-MDI and 4, 4' -MDI near the eutectic point of mixing. It has also been shown that the separated 2, 2' -rich MDI can be removed from interfering low boilers by repeated distillation and can be used for the preparation of polyurethanes and, as a result, its flammability can be reduced and the MDI yield can be increased.

Example 7 discloses, by way of an application example, the improved properties of polyurethane systems based on MDI products having a high 2, 4 '-MDI content and a minimized 2, 2' -MDI content.

Example 7：

This example is an example of the use of the MDI products of examples 1 to 4 according to the invention with a high 2, 4 '-MDI content (while minimizing the 2, 2' -MDI content) and products not according to the invention. An adhesive for two-sheet films was prepared and the amount of residual monomer discharged from the composite was used as a measure of reactivity. For composite membrane manufacturers, it is preferred that the total amount of effluent is as low as possible and that the storage time required for complete reaction is as short as possible.

Preparation of prepolymer I：

To 486 grams of the diphenylmethane diisocyanate prepared in example 1, having the following composition (commercially available from Bayer AG, 112mg KOH/g hydroxyl number and 0.03% by weight water content), was added 515 grams of the isocyanate-reactive material Desmophen1112BD ® at 60 ℃ under nitrogen:

52.74% by weight of 2, 4' -diphenylmethane diisocyanate,

1.79% by weight of 2, 2' -diphenylmethane diisocyanate, and

45.47% by weight of 4, 4' -diphenylmethane diisocyanate.

After the slightly exothermic reaction subsided, the reaction was terminated at 75 ℃. After 7 hours, a constant isocyanate content was obtained.

Analyzing data:

NCO content (wt%): 11.68

Viscosity at 25 ℃ (mPas): 5210

2, 2' -MDI content (wt%): 0.8

2, 4' -MDI content (wt%): 14.9

4, 4' -MDI content (wt%): 8.7

Preparation of prepolymer II

To 486 grams of diphenylmethane diisocyanate prepared in accordance with the process of the present invention as described in example 2 above, having the following composition, was added 515 grams of the isocyanate-reactive material Desmophen1112BD ® (commercially available from Bayer AG having a hydroxyl number of 112mg KOH/g and a water content of 0.03% by weight) under nitrogen at 60 deg.C:

52.52% by weight of 2, 4' -diphenylmethane diisocyanate,

0.07% by weight of 2, 2' -diphenylmethane diisocyanate, and

47.41% by weight of 4, 4' -diphenylmethane diisocyanate.

After the slightly exothermic reaction subsided, the reaction was terminated at 75 ℃. After 7 hours, a constant isocyanate content was obtained.

Analyzing data:

NCO content (wt%): 11.71

Viscosity at 25 ℃ (mPas): 4180

2, 2' -MDI content (wt%): < 0.05

2, 4' -MDI content (wt%): 15.1

4, 4' -MDI content (wt%): 9.5

Preparation of prepolymer III：

To 486 grams of the diphenylmethane diisocyanate prepared in example 3, having the following composition (commercially available from Bayer AG, 112mg KOH/g hydroxyl number and 0.03% by weight water content), was added 515 grams of the isocyanate-reactive material Desmophen1112BD ® at 60 ℃ under nitrogen:

95.12% by weight of 2, 4' -diphenylmethane diisocyanate,

3.37% by weight of 2, 2' -diphenylmethane diisocyanate, and

1.51% by weight of 4, 4' -diphenylmethane diisocyanate.

After the slightly exothermic reaction subsided, the reaction was terminated at 75 ℃. After 7 hours, a constant isocyanate content was obtained.

Analyzing data:

NCO content (wt%): 11.81

Viscosity at 25 ℃ (mPas): 4310

2, 2' -MDI content (wt%): 1.2

2, 4' -MDI content (wt%): 22.7

4, 4' -MDI content (wt%): < 0.05

Preparation of prepolymer IV：

To 486 grams of diphenylmethane diisocyanate prepared according to the process of the present invention as described in example 4, having the following composition, was added 515 grams of the isocyanate-reactive material Desmophen1112BD ® (commercially available from Bayer AG having a hydroxyl number of 112mg KOH/g and a water content of 0.03% by weight) at 60 ℃ under nitrogen:

97.05% by weight of 2, 4' -diphenylmethane diisocyanate,

0.16% by weight of 2, 2' -diphenylmethane diisocyanate, and

2.79% by weight of 4, 4' -diphenylmethane diisocyanate.

After the slightly exothermic reaction subsided, the reaction was terminated at 75 ℃. After 7 hours, a constant isocyanate content was obtained.

Analyzing data:

NCO content (wt%): 11.74

Viscosity at 25 ℃ (mPas): 4440

2, 2' -MDI content (wt%): < 0.05

2, 4' -MDI content (wt%): 23.0

4, 4' -MDI content (wt%): 0.4

Preparation of polyol mixtures：

1000 grams of Baycoll AD 1115 ® (having a hydroxyl number of 113.3mg KOH/g, an acid number of 0.8mg KOH/g, and a water content of 0.03 wt%) commercially available from Bayer AG, 80 grams of trimethylolpropane was vigorously mixed together. This gave a polyol having a hydroxyl number of 196.7 (mgKOH/g).

Preparation of polyurethane reaction mixtures and composite membranes:

by mixing 50 grams of the above polyol mixture together with the amount of prepolymer described in table 2. These amounts correspond to a characteristic value (NCO/OH or isocyanate index) of 140. The polyurethane reaction mixture was mixed vigorously with a wood spatula for 2 hours and then added to the nip of a Polytest440 laboratory laminator.

Table 2: preparation of polyurethane reaction mixtures A to D

PUR reaction mixture for composites	Amount of prepolymer (g)	Amount of polyol (g)
			A	88.3I	50
B	88.0II	50
			C	86.9III	50
D	87.8IV	50

The roll temperature was about 30 ℃ and the coating rate was about 10 m/min. The film width was 30 cm. The following membrane composites were prepared. The aluminum side of the polyester/aluminum pre-composite was bonded with a lubricant to LDPE-K-088(70 micron layer thickness) Low Density Polyethylene (LDPE) which had been corona treated to improve the adhesion of the bonding side. The amount of polyurethane reaction mixture applied is shown in table 3.

Composite Strength test

Samples were taken from laminates at least 20 meters long and 30cm wide wound on a sleeve. After each stretching of the composite film strip for 5 turns, a test specimen was cut in the middle. From the time of preparation, the composite film was stored in a room with a temperature controlled at 23 ℃ and an air humidity of 50%.

The composite strength tests were carried out in each case 24 hours, 3 days, 7 days and 14 days after the preparation of the composite membranes. For this purpose, a 15mm wide laminated tape was die cut to a length of 30cm with impact shearing parallel to the edges. The composite test was carried out according to the T-peel test of DIN53289 with a VNGG tester from Brugger, Munich at a peel rate of 100mm/min along a length of at least 10 cm. The data are given in newtons/15 mm. All results are the average of two measurements.

Table 3: results of composite Strength test

Composite material	Reaction mixtures as described in Table 2	Coating weight (g/m)2)	The compound strength in newtons/15 mm after the following days
										1	2	3	7	14	21	28
			A	I	2.8	5.6	8.8	10.3	10.8								10.8	11.8D	11.3D
B	II	3.0								5.0	9.2	10.0	9.2	11.0	11.6D	11.9D
			C	III	3.0	4.9	9.8	10.5	9.7								11.2D	11.2D	12.3D
D	IV	3.2								4.5	7.9	10.2	10.0	10.2	10.4D	12.5A

D ═ the film extension; a is the tearing of the film

Assessment of migration

The migration of aromatic polyamines is determined according to the method disclosed in "Amthiche Sammlung Ulterchungsverfahrenach § 35LBMG L.00.00-6" [ "Official Collection of Investigation method association to § 35LBMG L.00.00-6 ]".

The film composite thus measured was wound on a sleeve and stored in a chamber controlled at 23 ℃ and 50% relative humidity. The film rolls were opened in each case 10 rolls after 1, 3, 7 and 14 days, and then the two films were peeled off from one another. Sealing the membrane to form a film having a length of 2dm for the food to be tested²The contact area of (a).

The bag was filled with 100ml of 3% acetic acid preheated to 70 ℃, sealed and stored at 70 ℃ for 2 hours. Immediately after storage, the bags were emptied and the food was cooled to room temperature.

The polyurethanes which had migrated were determined by diazotization of the aromatic amine present in the test food, followed by coupling with N- (1-naphthyl) ethylenediamine. The diazo dye produced is then concentrated, eluted in a separation column and the extinction at 550 nm is measured in a photometer. With the aid of a calibration curve, the values are converted into micrograms of aniline hydrochloride per 100ml of test food. The error limit is given as 0.2 μ g aniline hydrochloride/100 ml.

Specific values are based on the average of three measurements.

Table 4: results of the migration numerical test

Composite material	The amount of amine found (expressed in micrograms aniline hydrochloride/100 ml) after the following days
							1	3	7	14	28	42
	A	14.8	2.7	1.35	0.83	1.23							1.0
B							11.3	0.83	＜0.2	＜0.2	＜0.2	＜0.2
	C	8.2	1.87	0.95	0.67	0.97							0.93
D							3.9	0.5	＜0.2	＜0.2	＜0.2	＜0.2

Table 4 shows that after 1 day and 3 days, the aromatic amine content found in the composite films which have been prepared from MDI mixtures having a high 2, 4 ' -MDI content (i.e. compounds C and D) is in each case lower than that of the comparative film compounds (i.e. compounds a and B) prepared from MDI mixtures having a lower 2, 4 ' -MDI content (i.e. containing 2, 2 ' -MDI in the amount used for comparison). Direct comparison of the film composites A and C and B and D is possible, depending on the comparative content of 2, 2' -MDI contained in the prepolymers used.

In addition, Table 4 shows that the film composites prepared with MDI prepolymers with low 2, 2' -MDI content (i.e.composites B and D) contain almost no aromatic amines after 7 days (i.e.below the detection limit in this test). On the other hand, aromatic amine concentrations above the detection limit could be found in film composites prepared with MDI prepolymers with high 2, 2' -MDI contents (i.e. composites a and C) even after 42 days.

It is thus evident that the diisocyanates of the diphenylmethane series with a low 2, 2 '-MDI content (i.e. the MDI fractions on which the composite films B and D are based) prepared by the process according to the invention have product properties in applications which exceed those of the diisocyanates of the diphenylmethane series with a high 2, 2' -MDI content (i.e. the MDI fractions on which the composite films A and C are based) prepared according to the prior art.

Although the invention has been disclosed 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 (18)

1. A process for the production of a polyurethane and/or prepolymer comprising reacting a polyisocyanate with one or more polyesters or polyethers, wherein the polyisocyanate comprises a fraction of 0 to 0.5 wt.% 2, 2 ' -MDI, 0.1 to 80 wt.% 4, 4 ' -MDI and 20 to 99.9 wt.% 2, 4 ' -MDI, based on the total weight of MDI isomers, the fraction containing at least 99 wt.% 2-nuclear methylene diphenyl diisocyanate based on the total weight of the fraction, the fraction being prepared by a process comprising the steps of:

a) reacting aniline and formaldehyde in the presence of an acid catalyst to form di-and polyamines of the diphenylmethane series comprising 2-nuclear methylenediphenyldiamine,

b) phosgenating di-and polyamines of the diphenylmethane series comprising 2-nuclear methylenediphenyldiamine, optionally in the presence of a solvent, thereby forming untreated di-and polyisocyanates,

c) separating a fraction comprising at least 95% by weight of 2-nuclear methylene diphenyl diisocyanate, based on the total weight of the fraction, of untreated diisocyanates and polyisocyanates, wherein the 4, 4 ' -MDI content is from 49 to 95.99% by weight, the 2, 4 ' -MDI content is from 4 to 45% by weight and the 2, 2 ' -MDI content is from 0.01 to 20% by weight,

d) optionally, 10 to 98% of the 4, 4' -MDI is removed from the fraction obtained in step c),

e) separating from the fraction obtained in step c) or step d) 50 to 99.9999 wt% of 2, 2 '-MDI, thereby producing a fraction comprising 0 to 0.4 wt% of 2, 2' -MDI, 1 to 95 wt% of 4, 4 '-MDI and 5 to 98.6 wt% of 2, 4' -MDI, based on the total weight of the MDI isomers.

2. The method of claim 1, wherein it further comprises:

f) separating a fraction comprising at least 99% by weight, based on the total weight of the fraction, of 2-nuclear methylene diphenyl diisocyanate from the fraction comprising 0 to 0.4% by weight of 2, 2 '-MDI, 1 to 95% by weight of 4, 4' -MDI and 5 to 98.6% by weight of 2, 4 '-MDI, based on the total weight of the MDI isomers, of 2-nuclear methylene diphenyl diisocyanate, comprising 0 to 0.5% by weight of 2, 2' -MDI, 0.1 to 80% by weight of 4, 4 '-MDI and 20 to 99.9% by weight of 2, 4' -MDI, based on the total weight of the MDI isomers, formed in step e).

3. The process of claim 1, wherein the fraction separated in step c) comprises at least 98% by weight of 2-nuclear methylene diphenyl diisocyanate, based on the total weight of the fraction, and has a 4, 4 ' -MDI content of 80 to 93.9% by weight, a2, 4 ' -MDI content of 6 to 19.9% by weight and a2, 2 ' -MDI content of 0.1 to 5% by weight.

4. The process of claim 1, wherein in step d) 4, 4' -MDI is removed from the fraction in a distillation step or a crystallization step.

5. The process of claim 4, wherein 50-95% of the 4, 4' -MDI is removed from the fraction in step d).

6. The process of claim 1, wherein the 2, 2 '-MDI separated in step e) is purified by crystallization or distillation to remove low boiling compounds, thereby producing an isomeric mixture comprising 5 to 99.99 wt.% 2, 2' -MDI, 0 to 50 wt.% 4, 4 '-MDI and 0.01 to 95 wt.% 2, 4' -MDI, based on the total weight of the MDI isomers.

7. The process of claim 6, wherein the resulting isomer mixture comprising 5 to 99.99 wt% 2, 2 ' -MDI, 0 to 50 wt% 4, 4 ' -MDI and 0.01 to 95 wt% 2, 4 ' -MDI, based on the total weight of MDI isomers, is optionally mixed with other fractions comprising isocyanates and then reacted with a polyether or polyester to form a polyurethane or prepolymer.

8. The process of claim 1, wherein step e) produces a fraction comprising 0 to 0.3 wt% 2, 2 ' -MDI, 5 to 85 wt% 4, 4 ' -MDI and 15 to 95 wt% 2, 4 ' -MDI, based on the total weight of the MDI isomers.

9. The process of claim 2, wherein the separation of step f) is carried out by distillation at an absolute pressure of from 0.5 to 30 mbar.

10. The process of claim 9, wherein the distillation is carried out at an absolute pressure of 1 to 15 mbar.

11. The process of claim 2, wherein the fraction obtained in step f) comprises 0 to 0.35 wt.% of 2, 2 ' -MDI, 0.2 to 60 wt.% of 4, 4 ' -MDI and 40 to 99.8 wt.% of 2, 4 ' -MDI, based on the total weight of the MDI isomers.

12. The process according to claim 2, wherein the fraction obtained in step f) comprises a fraction of 0 to 0.2 wt.% of 2, 2 ' -MDI, 0.5 to 55 wt.% of 4, 4 ' -MDI and 45 to 99.5 wt.% of 2, 4 ' -MDI, based on the total weight of the MDI isomers.

13. The process of claim 1, wherein steps d) and e) are performed simultaneously in a common distillation step.

14. The process of claim 13, wherein the general distillation step is performed in a side-stream distillation column.

15. The process of claim 2 wherein steps e) and f) are carried out simultaneously in a common distillation step.

16. The process of claim 15, wherein the general distillation step is performed in a side-stream distillation column.

17. The process of claim 6, wherein the separation in step f) and the separation of the low-boiling compounds from 2, 2' -MDI in step e) are carried out simultaneously in a common distillation step.

18. The process of claim 17, wherein the general distillation step is performed in a side stream distillation column.