MXPA00010183A - Method for producing mixtures consisting of diphenylmethane diisocyanates and polyphenylene-polymethylene-polyisocyanates containing a reduced amount of chlorinated secondary products and with a reduced iodine colour index - Google Patents

Abstract

The invention relates to a method for producing mixtures consisting of diphenylmethane diisocyanates and polyphenylene-polymethylene-polyisocyanates containing a reduced amount of chlorinated secondary products and with a reduced iodine colourindex. According to said method, a two-stage reaction of the corresponding mixtures of diphenylmethane-diamines and polyphenylene-polymethylene polyamines with phosgene is carried out in the presence of at least one inert organic solvent at a high temperature. When the phosgenation is complete, the excess phosgene and solvent are separated off and the product of the reaction is treated with heat. The method is characterised in that the mass ratios of phosgene to hydrogen chloride in the detention apparatus in the second stage of phosgenation are simultaneously 10-30:1 in the liquid phase and 1-10:1 in the gas phase.

Description

PREPARATION OF DIPHENYLMETHANE DIISOCYANATE MIXTURES AND POLIISO POLYANETHYLENE POLYISOCIANATES WITH A CONTENT REDUCED CHLORINE BYPRODUCTS AND A REDUCED INDEX OF IODINE COLOR The present invention relates to a process for the preparation of mixtures of diphenylmethane diisocyanates and polyphenylenepolymethylene polyisocyanates, known as PMDI, which has a reduced content of chlorinated by-products and a low color index of iodine, by reaction in two stages of the corresponding mixtures of diphenylmethane diamines and polyphenylpolymethylenepolyamides, known as PMDA, with phosgene in the presence of at least one inert organic solvent, where the corresponding carbamoyl chlorides formed in the first stage of phosgenation and amine hydrochlorides in the second stage of phosgenation run through of an apparatus with a residence time in which the amine hydrochlorides are phosgenated to the corresponding carbamoyl chlorides and the carbamoyl chlorides are dissociated in the corresponding isocyanates of the acid chloride and the mass ratios of phosgene to acid chloride are the same 10-30: 1 time in the liquid phase and 1-1 0: 1 in the gas phase. PMDI is the industrially most important isocyanate for the manufacture of rigid polyurethane foams that They are preferably used as insulation material in the construction industry, as insulating foam in refrigeration industry applications and as a material intercalated in the construction, usually part of diphenylmethane 4, '-diisocyanate, known as MMDI, present in PMDI is recovered by means of an appropriate technological operation, such as distillation or crystallization. MMDI in turn is an important constituent of polyurethane formulations for compact, microcellular and cellular polyurethanes, such as adhesives, coatings, fibers, elastomers and integral foams. Accordingly, the term "PMDI" used in the present text also encompasses PMDI mixtures in which the monomeric MDI, for example, '-, 2,' - and / or, '-MDI is present. PMDI, as is known, is prepared by phosgenation of the corresponding PMDA in the presence of an inert organic solvent. The PMDA, in turn, is obtained by means of a condensation of an acid-formaldehyde aniline which can be carried out on an industrial scale in a continuous or batchwise manner. The proportions of diphenylmethane diamines and the homologs polyphenylpolymethylenepolyamines and their positional isomerism in the PMDA are controlled by selection of the ratios aniline, formaldehyde and acid catalyst and also by means of an appropriate temperature profile and residence time. The high contents of diphenylmethanediamine together with a low proportion of 2-isomer of diphenylmethanediamine are simultaneously obtained on an industrial scale by the use of strong mineral acids such as hydrochloric acid as a catalyst in the aniline-formaldehyde condensation. All the acid condensation processes of the aniline-formaldehyde described in the patent and specialist literature have in common the formation of unwanted by-products, for example, the formation of N-methylated and N-formylated compounds and also the formation of dihydroquinazolms. In addition, industrial PMDA may contain residual amounts of ammobenzylaniline without rearrangement that may, in turn, be a starting point for further reactions. Another disadvantage is that the acid condensation of anilma-formaldehyde forms croraphores that discolor the PMDA. These discolorations are insufficiently reduced, if anything, in the subsequent neutralization of the acid condensation catalyst and the separation of the aniline used in excess in the condensation; The same applies to the subsequent steps of the PMDI preparation process. In the phosgenation step, the PMDA is reacted with phosgene in an inert organic solvent to form PMDI. The unwanted byproducts and chromophores in the PMDA can react with phosgene to form more compounds, such as secondary carbamoyl chlorides and chlorination products of the aromatic ring and / or methylene bridge. In addition, the phosgenation step also forms chlorine-containing byproducts such as alofanoyl chlorides and isonitrile dichlorides. The compounds containing chlorine and chromophores are incorporated in the low molecular weight fraction, whose central constituent is diphenylmethane dusocyanate and also in the oligomeric fractions of polyphenylenepolymethylene polusocyanate. The technological operations that follow phosgenation, namely the separation of the excess phosgene used, the separation of the inert solvent, the heat treatment, the so-called dechlorination and the separation of part of the MMDI present in the crude PMDI by distillation and / or crystallization, do not permanently reduce the content of chlorine-containing compounds and the discoloration of crude PMDI increases with continuous effort of the product, especially thermal The discolored and / or chlorine-containing PMDI is undesirable in further processing to form polusocyanate plastic polyadicion plastics -polialcohol. In particular, chlorine-containing compounds that can rapidly form ionic chloride, as determined by ASTM method D 1638-74, can cause considerable interference in the blowing reaction in the production of foam forming salts with the blowing catalyst. Undesirable discolorations of PMDI also appear in the plastics prepared from these. Although the color of polusocyanate-polyalcohol polyaddition plastics do not have an adverse effect on their mechanical properties, the slightly colored products are preferred due to their good versatility in the production processes of the processor, for example, the possibility of light to pass through through thin layers for covers and the possibility of producing a variety of colors. Therefore, there have been many attempts to reduce the content of chlorinated byproducts and the discoloration of PMDI in mixtures with MMDI. According to GB 1 549 294, the addition of isoureas in an amount of 25-250 mol% can reduce the acidity ASTM D 1638-74 of the PMDI. A disadvantage of this method is that an additional agent has to be used and the decrease in acidity is only partially successful. DD 285 593 proposes the treatment of PMDI with acid amides in an amount of 0.01-0.2% at 100-140 ° C for 0.2-6 hours. After the treatment, the acid chloride formed is removed by stripping with the nitrogen or solvent vapors. The disadvantages of this process are the insufficient effect of the acid amides, the formation of additional constituents in the PMDI as a result of the inevitable secondary reaction of the isocyanates with the acid amides to form acylated ureas and the expense in terms of the apparatus for treatment of the PMDI with the acid amides and for the entrainment of the acid chloride, which was added as a catalyst and the one that was formed. DE 2 847 243 proposes the separation of phosgene by entrainment with gaseous acid chloride or nitrogen at 170 ° C for 2 hours. One disadvantage is the considerable amounts of gases charged with phosgene or with phosgene acid chloride which makes an additional expense for the subsequent separation of materials or an additional expense for the neutralization of the acid gas, absolutely necessary constituents. The additional disadvantage of the process described in DE 2 847 243, namely the prolonged residence time for entrainment, is partially relieved in JP 07 233 136A by a two-step entrainment of acid chloride after separation of the phosgene at 115 ° C / 30 minutes and 160 ° C / 3 minutes. However, this causes the disadvantage of an additional technological operation and again a significant gas demand that requires treatment. According to JP 07 082 230A, organic phosphites are added to the aniline before aniline-formaldehyde condensation.

To reduce the color index of iodine, the addition of numerous compounds after phosgenation has been proposed: water (US 4 465 639) phenol derivatives (DE 4 300 774), amines and / or ureas (DE 4 232 769) , acid chlorides / chloroformates (DE 4 118 914), polyoxyalkylene polyalcohols (DE 4 021 712), dialkyl or trialkyl phosphites (DE 4 006 978), low molecular weight monohydric or polyhydric alcohols (EP 445 602), acid chlorides / antioxidant (DE 4 318 018). All the processes that propose the addition of the compounds to the raw materials or products of a preparation stage for PMDI have the disadvantage of the addition of an additional agent with the inherent danger of its corrosive action on the components of the equipment and the formation of by-products from precisely these added agents, whose by-products can, in turn, have an adverse effect on the product or the equipment. Patent US 4 876 380 proposes the color clarification by extraction of a PMDI fraction r ca in chromophore from the PMDI by means of pentane / hexane. Advantages of this process are the complicated technological operation used with additional steps to treat the extract and the inevitable formation of a fraction of PMDI of reduced quality for which applications that consume equivalent quantities have to be found.

It is an object of the present invention to reduce the content of chlorinated by-products and the color index of the PMDI iodine in mixture with MMDI while avoiding the aforementioned disadvantages. In particular, the addition of auxiliaries and / or the use of additional devices should not be necessary. We have found that this object is achieved by reaction in two stages of the corresponding mixtures containing diphenylmethanediamines and polyphenylpolymethylenepolymas with phosgene in the presence of at least one inert organic solvent, where the corresponding carbamoyl chlorides formed in the first stage of the phosgenation and the hydrochlorides of amines in the second stage of the phosgenation run through a residence time apparatus in which the amine hydrochlorides are phosgenated to the corresponding carbamoyl chlorides and the carbamoyl chlorides are dissociated into the corresponding isocyanates and acid chloride and the Phosgene mass ratios for acid chloride are at the same time 10-30: 1 in the liquid phase and 1-10: 1 in the gas phase. Accordingly, the present invention provides a process for the preparation of mixtures containing diphenylmethane dusocyanates and polyphenylenepolymethylene polusocyanates having a reduced content of chlorinated byproducts and a reduced color index of iodine. by reaction in two stages of the corresponding mixtures containing diphenylmethanediamamines and polyphenylenepolymethylene polyamines with phosgene in the presence of at least one inert organic solvent, wherein the corresponding carbamoyl chlorides formed in the first stage of the phosgenation and the amine hydrochlorides in the second stage stage of the phosgenation run through an apparatus with time of resistance in which the amine hydrochlorides are phosgenated to the corresponding carbamoyl chlorides and the carbamoyl chlorides are dissociated into the corresponding isocyanates and acid chloride and the phosgene mass ratios for acid chloride are at the same time 10-30: 1 in the liquid phase and 1-10: 1 in the gas phase. The phosgenation of primary amines in a mixing reactor as the first stage of phosgenation has been described a number of times. Thus, for example, US Patent 3 544 611 and EP A2-0150435 report phosgenation in a pressure mixing circuit. In addition, EP A2-0291819 discloses carrying out this reaction in a reaction pump. Many different designs of static mixers have been described, for example, the nozzle with ring groove (FR 2 325 637, DE 1 792 660), ring eye nozzle (DE 3 744 001), flat jet nozzle (EP Al- 0 065 727), fan jet nozzle (DE 2 950 216), nozzle with angular jet chamber (DD 300 168), nozzle of three fluids (DD 132 340). As is known, per se, that the corresponding carbamoyl chlorides and the amine hydrochlorides formed in the first stage of the phosgenation can run through a residence time apparatus in which the amine hydrochlorides are phosgenated to form the corresponding Carbamoyl chlorides and the carbamoyl chlorides are dissociated into the corresponding isocyanates and acid chloride. Isocyanates prepared according to WO 96/16028 in a reactor tube at 80-150 ° C have a very unsatisfactory hydrolyzable chlorine content of max. 2% and make the PMDI prepared by this process unusable for many applications. In BE 790 461 and BE 855 235, agitator devices are used as residence time reactors. US 3 544 611 discloses a residence time distillation apparatus operating at 10-50 bar and 120-150 ° C and having an "elongated distillation zone" to dissociate the carbamoyl chlorides and separate the acid chloride. DE 3 744 001 proposes a column of perforated plates through which the reaction mixture flows from the rising bottom and which has more than 10 perforated plates, a residence time of max. 120 minutes and liquid speeds of 0.05-4 m / s and gas velocities of 2-20 m / s. The disadvantages of the prior art are the drastic conditions in the apparatus of the residence time and the relatively long time of residence of the crude PMDI formed. Experience indicates that the prior art allows only a very unsatisfactory quality level with respect to the color and chlorine content of the PMDI. The combination of mixing apparatus and residence times for the preparation of PMDI is also known, in particular for two-stage phosgenation. Thus, in DE 3 744 001, one with an annular nozzle as reactor for the reaction of primary amines with phosgene in an inert solvent to give the corresponding carbamoyl chlorides and amine hydrochlorides are combined with one or more columns of plates perforated as apparatuses for the phosgenation of amine hydrochlorides and dissociation of carbamoyl chlorides. In US 3 381 025, the first step is carried out at < 60 ° C in an inert solvent having a boiling point of 100-190 ° C and the reaction product is transferred to a second stage in which the temperature is maintained up to such a level above the boiling point of the inert solvent that the The phosgene escape ratio to the inert solvent is greater than 2 and, if desired, the phosgenation is further fed into the second reaction step. The disadvantages are the high cost in terms of appliances and high energy consumption in the second stage of the phosgenation as residence time apparatus or to condense the gas mixture of phosgenation / inert solvent. Experience indicates that the prior art allows only a very unsatisfactory quality level with respect to the chlorine content and the PMDI color. Therefore, it is a further object of the present invention to reduce the content of chlorinated byproducts and the iodine color index of the PMDI using technological equipment that is simpler in terms of safety and appliances. We have found that this object is achieved by the two-step reaction of MPDA with phosgene in the presence of at least one inert solvent, wherein the first step of the phosgenation is carried out in a static mixer and the second stage of phosgenation is carried out. carried out in a residence time apparatus and the phosgene mass ratios for acid chloride in the residence time apparatus are at the same time 10-30: 1 in the liquid phase and 1-10: 1 in the gas phase . The static mixers used in the first stage of the phosgenation are known and the aforementioned pieces of equipment, in particular nozzles. The temperature in the first stage of the phosgenation is usually from 40 to 150 ° C, preferably from 60 to 130 ° C, particularly preferably 90-120 ° C. The mixture of the first step of the phosgenation is fed to a residence time apparatus in which, according to the present invention, the ratio of phosgene mass to acid chloride in the second stage of phosgenation is at the same time 10-30: 1 in the liquid phase and 1-10: 1 in the gas phase. Apparatus with residence time used for the processes of the present invention are the known apparatuses, preferably apparatus with agitator, in particular cascades of container with agitator having from 2 to 6 vessels with agitator, or columns, in particular those having < 10 theoretical dishes. When using agitator apparatuses as residence time apparatuses, use is made, in particular as indicated above, of cascades of agitator vessels having at least 2, preferably from 2 to 6, particularly preferably from 2 to 5. containers with agitator. In principle, it is also possible to use cascades that have more than 6 vessels with agitator, but the increase in the number of vessels with agitator above 6 only serves to increase the cost in terms of the device without obtaining a measurable improvement in the product The mixture from the first stage of the phosgenation usually enters the first apparatus with agitator at 70-120 ° C, preferably 85-105 ° C. The temperatures in the apparatus with agitator are preferably, all together or individually different, 75-120 ° C, particularly from Preference 80-110 ° C The pressures in the agitator apparatus are usually, individually different or all together, 1.0-3.0 atm (pressure manometpca), preferably 1.2-2.5 atm (pressure manometpca). Particular preference is given to the use of a column as a residence time apparatus. Here, it is particularly advantageous to operate the column countercurrently. The product mixture of the first stage of the phosgenation is preferably fed into the column so that the mixture of PMDI / solvent / phosgene exits at the bottom of the column and a mixture of phosgene / acid chloride is separated at the top of the column and is fed to the separation of phosgene / acid chloride. The temperature at which the mixture of the first stage of the phosgenation enters the column is preferably 80-120 ° C, particularly preferably 82-117 ° C. The temperature at the bottom of the column is preferably 80-120. ° C, particularly preferably 90-110 ° C. The pressure in the upper part of the column is preferably 1.0-4.7 atm (manometric pressure), particularly preferably 2.0-3.7 atm (manometpca pressure). The ratio of acid chloride / phosgene in the column is adjusted and controlled by means of excess phosgene in the first stage of the phosgenation, the temperature at which the reaction product enters the column, the column pressure and the temperature at the bottom of the column.

All the phosgene can be fed in the first stage of the phosgenation or only part of it can be introduced in the first stage. In the latter case, an additional amount of phosgene is fed into the residence time apparatus of the second stage of phosgenation. The column used preferably has < 10 theoretical dishes. The use of a tray column with valve is advantageous. It is also possible to use other columns with internal attachments which improve the residence time necessary for the dissociation of carbamoyl chloride and the rapid and effective separation of the acid chloride, for example, columns of bubble cap trays or distillation trays having landfills of relatively high liquid. The column of perforated plates proposed in DE-A 3 744 001 is very inadequate in the industry for the task of light dissociation of carbamoyl chlorides together with the rapid and effective separation of acid chloride and is inadequate as a residence time apparatus for prepare a PMDI that has a reduced chlorine content and a reduced iodine color index due to its co-current principle that inevitably leads to large liquid deposits and the greater difficulty in achieving the rapid separation of the acid chloride. The mixtures of dusocyanates of diphenylmethane and polyphenylpolymethylene polusocyanates, prepared by the process of the present invention, usually have a content of the diphenylmethane dusocyanate isomer from 30 to 90% by mass, preferably from 30 to 70% by weight, an NCO content from 29 to 33% by weight, preferably from 30 to 32% by weight. % by mass, based on the weight of the raw MDI, and a viscosity, determined at 25 ° C in accordance with DIN 51550, of not more than 2500 mPa.s. preferably from 40 to 2000 mPa.s. Crude MDI compositions having such isomer and homologous compositions can be prepared by phosgenation of crude MDA compositions having corresponding product compositions in the presence of at least one inert organic solvent. Suitable crude MDAs are advantageously obtained by condensation of aniline and formaldehyde in a molar ratio of 6-1.6: 1, preferably 4-1.9: 1, and a molar ratio of aniline for acid catalysts of 1: 0.98-0.01, preferably 1: 0.8-0.1. The formaldehyde is preferably used in the form of an aqueous solution, for example, as a 30-50% commercial solution of bulk concentration. Acid catalysts which have been found useful are proton donors, such as acid ion exchange resins, organic acids and preferably strong inorganic acids. For the purposes of this invention, strong acids are those that have a pKa of less than 1.5; in the case of polybasic acids, this value is for the first dissociation of hydrogen. Examples that may be mentioned are hydrochloric acid, sulfuric acid, phosphoric acid, fluorosulphonic acid and oxalic acid. Acid chloride in gaseous form can also be used. Preference is given to the use of aqueous acidic hydrochloride in concentrations from about 25 to 33% by mass. Suitable processes for the preparation of crude MDA are described, for example, in CA-A-700 026, DE-B-22 27 110 (US-A-4 025 557), DE-B-22 38 920 (US Pat. A-3, 996.283), DE-B-24 26 116 (GB-A-1, 450, 632), DE-A-12, 2, 623 (US-A-3,478,099), GB-A-1,064,559 and DE -A-21 25 125. The other initial component for the preparation of crude MDI is phosgene. The gaseous phosgene can be used as such or diluted with gases that are inert under the reaction conditions, for example, nitrogen, carbon monoxide, and the like. The molar ratio of crude MDA to phosgene is advantageously selected such that from 1 to 10 mol, preferably from 1.3 to 4 mol, of phosgene are present in the reaction mixture per mole of NH2 groups. The phosgene can all be fed in the first stage of the phosgenation or part of it can also be added in the apparatus of the residence time of the second stage of phosgenation. Suitable inert organic solvents are compounds in which the crude MDA and the phosgene are at least partially soluble. Solvents which have been found useful are chlorinated aromatic hydrocarbons, for example monochlorobenzene, dichlorobenzene such as o-dichlorobenzene and p-dichlorobenzene, trichlorobenzene, the corresponding toluenes and xylenes, chloroethylbenzene, monochlorobiphenyl, alpha- or beta-naphthyl chloride and dialkyl phthalates such as diethyl isophthalate . Particular preference is given to the use of monochlorobenzene, dichlorobenzene or mixtures of these chlorobenzenes as inert organic solvents. The solvents can be used individually or as a mixture. It is advantageous to use a solvent having a lower boiling point than that of the MDI isomers so that the solvent can be easily separated from the raw mMDI by distillation. The amount of solvent is advantageously selected so that the reaction mixture has a content of socianates from 2 to 40% by mass, preferably from 5 to 20% by mass, based on the total weight of the reaction mixture. The raw MDA can be used as such or as a solution in organic solvents. However, particular preference is given to using raw MDA solutions that have an amine content from 2 to 45% by mass, preferably from 25 to 44% by mass, based on the total weight of the amine solution. After phosgenation, the excess phosgene, the acid chloride and the solvent are preferably separated from the reaction product. To prepare a PMDI having a reduced content of chlorinated by-products and a reduced color index of iodine, it is particularly advantageous because the residual phosgene content after separating the phosgene is < 10 ppm phosgene. These steps are gradually carried out by generally known methods. The two-ring isomers can be separated from the MDI mixture by methods such as distillation or crystallization.

The product is then stabilized using an antioxidant based on spherically hindered phenols and / or at least one apl phosphite. The stabilizers are advantageously used in an amount up to max. 1% by mass, preferably from 0.001 to 0.2% by mass. Examples of suitable antioxidants based on spherically hindered phenols are: styrene-stabilized phenols, that is, phenols having a 1-phenol group joined at position 2 or 4 or at positions 2 and 4 and / or 6, bis (2-hydroxyl-5-methyl-3-tert-butyl-phenyl) methane, 2,2-bis (4-hydroxyphenyl) propane, 4,4'-dihydroxybiphenyl, 3,3'-dialkyl-3 , 3 ', 5, 5' -tetraalkyl-4, 4'-dihydroxybiphenyl, bis (4-) hydroxyl-2-methyl-5-tert-butylphenol) sulfide, hydroquinone, 4-methoxy-, 4-tert-butoxy- or 4-benzyl-phenol, mixtures of -metox? -2- or 3-tert-but-lphenol, 2, 5-d? h? drox? -l-tert-butylbenzene, 2, 5-d? h? drox? -l, 4-d? - tert-butylbenzene, 4-methoxy, -2,6-d-tert-but-l-phenol and preferably 2,6-d-tert-butyl-p-cresol. The aryl phosphites found to be useful are tri- (alkyl-phenyl) phosphites having from 1 to 10 carbon atoms in the alkyl radical, for example, tp (methylphenyl) phosphite, tri (ethylphenyl) phosphite, tri (n-) propylphenyl) phosphite, tp (isopropylphenyl) phosphite, tr? (n-butylphenyl) phosphite, tp (sec-butylphenyl) phosphite, tp (tert-butylphenyl) phosphite, tri (pentylphenyl) phosphite, tp (hexylphenyl) phosphite, tri (2) -et? lhex? lfen? l) phosphite, tp (octylphenyl) phosphite, tri (2-et? loct? lfen? l) phosphite, tp (decylphenyl) phosphite, and preferably tp (nonylphenyl) phosphite, and in particular triphenylphostium. The present invention also provides a process for preparing 2,2'-, 2,4'- and / or 4,4'-MDI from the mixture comprising diphenylmethane dusocyanate and polyphenolpolymethylene polusocyanate prepared in accordance with the present invention, which comprises separating 2,2'-, 2,4'- and / or 4,4'-MDI, preferably 4,4'-MDI by distillation and / or crystallization of the mixtures prepared according to the present invention.

Accordingly, raw PMDIs prepared in this way are normally subjected to a subsequent heat treatment which can be coupled with the separation of the MMDI isomers. For this purpose, the MPDI is heated to 170-230 ° C, preferably 180-220 ° C, and treated at this temperature at a pressure of 0.01 to 100 mbar, preferably from 0.1 to 20 mbar, for at least 5 minutes and in particular from 5 to 45 minutes, if desired while passing an amount of not more than 5 m3 / t standard of PMDI of an inert gas such as nitrogen, preferably not more than 0.5 m3 / t standard of PMDI of inert gas. After cooling to 30-60 ° C, the MPDI normally passes to an intermediate storage. The invention is illustrated by the following examples: Example 1: The phosgenation is carried out using a PMDA having the following composition: - viscosity at 70 ° C 348 mm2 / s - content of 4, 4 '-diphenylmethanediamine - (4,4'-MDA) 44.6% by mass - MDA content 52% by mass - PMDA content of 3 rings 23% by mass - content N-methyl-MDA 0.14% by mass - N-formyl-MDA content 1194 ppm. 3840 kg / h of a PMDA as a solution at 38.7% concentration in mass in monochlorobenzene (MCB) are phosgeneated with 26,400 kg / h of phosgene solution at 42% concentration in MCB in a nozzle with angular jet chamber. The reaction mixture was heated to 118 ° C in the reactor of the first stage of the phosgenation as a result of the exothermic reaction of PMDA with phosgene and is at 92 ° C at the inlet in a column of trays with valve having 6 Theoretical plates in the separation section and two plates in the enrichment section. The column operates at a pressure of 4.3 bar (ABS) and the composition of the waste is adjusted by means of the amount of steam used for heating so that the phosgene content at the bottom of the column is about 10 mass%, which corresponds to a temperature at the bottom of the column of 95-97 ° C. The phosgene mass ratios for acid chloride are 14.2: 1 at the bottom of the column and 1.6: 1 at the top of the column. The acid chloride formed in the first stage of the phosgenation and released in the dissociation column of the carbamoyl chlorides, together with the phosgene part used in excess, it is removed from the top at 91 ° C.

To prevent the entry of PMDI droplets into the acid chloride and phosgene gas streams, 1350 kg / h of MCB is further fed into the top of the column. The mixture leaving the phosgenation is freed from phosgene and MCB and then heat treated according to the prior art. The PMDI prepared in this way has the following properties of the product: - viscosity at 25 ° C in accordance with DIN 51550 182 mPa. s - isocyanate group content according to ASTM D 1638-74 31.5% by mass - acidity according to ASTM D 1638-74 56 ppm HCl - total chlorine according to DIN 35474 900 ppm HCl - iodine color index1) 9.7 1! Measured using a 3-filter instrument, for example. LICO 200 (Dr. Lange).

Comparative Example 1: For comparison, the same PMDA as in Example 1 was Phosgene in the same nozzle with angle injection chamber and the same column. 3849 kg / h of this PMDA as a solution at 38.7% mass concentration in monochlorobenzene (MCB) is likewise reacted with 26,400 kg / h of a phosgene solution of 42% concentration by mass in MCB. In the same way, 1350 kg / h of MCB are additionally fed in the upper part of the column. The temperature at the input of PMDA / MCB current in the nozzle with angle injection chamber is selected so that the temperature of the reaction mixture leaving the nozzle is 96 ° C. The reaction mixture is 78 ° C at the inlet of the valve tray column. The column operated at a pressure at the top of 5.2 bar (abs.). At a temperature in the funds set to 116 ° C, a temperature in the upper part of 76 ° C is established. The phosgene mass ratios for acid chloride are 9.2: 1 at the bottom of the column and 0.95: 1 at the top of the column. The PMDI prepared as a comparison has the following properties of the product: - viscosity at 25 ° C according to DIN 51550 197 mPa. s - isocyanate group content according to ASTM D 1638-25 74 31.8% by mass - acidity in accordance with ASTM D 1638-74 197 ppm HCl - total chlorine according to DIN 35474 1900 ppm HCl - iodine color index1) 15

Claims (1)

CLAIMS A process for preparing mixtures containing diphenyl methane dusocyanates and polyphenylene polymethylene polusocyanates having a reduced content of chlorinated by-products and a reduced color index of the iodine by reaction in two stages of the corresponding mixtures, containing diphenylmethane amyramides and polyphenylenepolymethylene polyamines with phosgene in the presence of at least one inert organic solvent, wherein the phosgene mass ratios for acid chloride in the residence time apparatus of the second stage of phosgenation are at the same time 10: 30: 1 in the liquid phase and 1-10: 1 in the gas phase. The process, as mentioned in claim 1, wherein the apparatus used in the first step of the phosgenation is a static mixer having an outlet temperature of the mixture of 80-120 ° C. The process, as mentioned in claim 1, wherein the apparatus used in the second step of the phosgenation is a column having < 10 theoretical dishes. The process, as mentioned in claim 3, wherein the column operates counter-current. The process, as mentioned in claim 3, wherein the column is a tray column with valve. The process, as mentioned in claim 3, wherein the column is a column of trays with a bubble cap. The process, as mentioned in claim 3, wherein the column has distillation trays that have relatively high liquid spillways. The process, as mentioned in claim 1, wherein the apparatuses used in the second stage of the phosgenation are from 2 to 6 agitator apparatuses connected in series. The process, as mentioned in claim 1, wherein the apparatuses used in the second stage of the phosgenation are from 2 to 5 agitator apparatuses connected in series. The process, as mentioned in claim 1, wherein the concentration of PMDA in the inert solvent in the stream to the static mixer is not more than 44% by mass. The process, as mentioned in claim 3, wherein the temperature at which the mixture of the first step of the phosgenation enters the column is 80-120, preferably 82-117 ° C.

1 . The process, as mentioned in claim 3, wherein the temperature at the bottom of the column is 80-120 ° C, preferably 90-110 ° C. The process, as mentioned in claim 3, wherein the pressure at the top of the column is 1.0-4.7 atm (manometric pressure), preferably 2.0-3.7 atm (manometric pressure). The process, as mentioned in claim 4, wherein the temperature at which the mixture of the first step of the phosgenation enters the first agitator apparatus is 70-120 ° C, preferably 85-105 ° C. 15. The process, as mentioned in claim 4, wherein the temperatures in the apparatus with agitators are, individually different or together, 75-120 ° C, preferably 80-110 ° C. 16. The process, as mentioned in claim 4, wherein the pressures in the apparatuses are, individually different or together, 1.0-3.0 atm (manométpca pressure), preferably 1.2-2.5 atm (manométpca pressure). 17. A mixture containing diphenylmethane dusocyanates and polyphenylenepolymethylene polusocyanates obtained by a process as mentioned in any of claims 1 to 15. 18. A process for preparing 2,2'-, 2,4'-, and / or 4, 4'-MDI a part of a mixture comprising diphenylmethane dusocyanate and polyphenylenepolymethylene polusocyanate, which consists of: separating 2,2'-, 2,4'-, and / or 4,4'-MDI, preferably 4,4'-MDI, a from a mixture as mentioned in claim 16 by distillation and / or crystallization.