Organic compounds -- part of the class 532-570 series – Organic compounds – Isocyanate esters
Reexamination Certificate
2000-09-08
2002-08-13
Gorr, Rachel (Department: 1711)
Organic compounds -- part of the class 532-570 series
Organic compounds
Isocyanate esters
C560S358000, C560S359000, C564S330000, C564S331000, C564S333000, C564S334000
Reexamination Certificate
active
06433219
ABSTRACT:
The present invention relates to a process for preparing methylenedianiline by reacting aniline with formaldehyde in the presence of acid catalysts, the mixtures which can be prepared by this process comprising methylenedianiline, a process for preparing polyisocyanates by phosgenation of these mixtures comprising methylenedianiline, and polyisocyanates obtainable in this manner.
The preparation of methylenedianiline (also termed MDA below), is generally known and is customarily carried out by continuous or batchwise reaction of aniline with formaldehyde in the presence of acid catalysts. In this reaction, whose main product is 4,4′-MDA, the unwanted byproduct N-methyl-MDA is formed to a small extent. This byproduct is disadvantageous, in particular in the subsequent reaction of the MDA with phosgene to prepare methylenebis(phenyl isocyanate), also termed MDI, since the N-methyl-MDA is the precursor compound for chlorinated byproducts in the MDI and chlorine contents in the MDI as low as possible are sought.
To decrease N-methyl-MDA as byproduct in the preparation of MDA, various processes are known.
Thus, U.S. Pat. No. 5,286,760, for continuous MDA preparation, describes partial neutralization of the reaction mixture between the stage of condensation of two molecules of aniline and one molecule of formaldehyde and the subsequent rearrangement of the intermediate aminobenzylamines, abbreviated as ABA, to give MDA. EP-A 451 442 and DD-A 238 042 disclose, for a continuous process, the addition of formaldehyde over a plurality of process stages. Processes for decreasing the byproduct are also known for batchwise processes. DD-A 295 628 describes the addition of formaldehyde in two steps during the condensation stage, in the first addition the main amount of formaldehyde being added at a low temperature and the second addition of the remaining formaldehyde being performed at the same or higher temperature.
A disadvantage in these processes is the insufficient decrease of the N-methyl-MDA content in the product mixture, so that there is still a need for improvement.
Processes for preparing MDI from MDA by phosgenation are generally known.
It is an object of the present invention to develop a process for preparing methylenedianiline by reacting aniline with formalde-hyde in the presence of acid catalysts which minimizes the N-methyl-MDA content as an unwanted byproduct. Such an MDA should be used, in particular, in an improved process for preparing methylenebis(phenyl isocyanate) (MDI), which should make accessible an MDI having improved properties, in particular a low chlorine content and/or a light color, in particular in the crude MDI which, in addition to the monomeric MDI, also comprises polymeric MDI, and/or should be made accessible in the polymeric MDI.
We have found that this object is achieved according to the invention, in a semicontinuous process, by introducing aniline with or without acid catalyst, feeding formaldehyde with or without acid catalyst through a mixing element into a circuit in which aniline with or without acid catalyst and with or without previously added formaldehyde is circulated and, after feeding in at least 50% of the total amount of formaldehyde to be fed in, heating the reaction mixture to a temperature above 75° C.
This novel procedure permits a higher content of higher MDA oligomers to be obtained than is possible by a continuous procedure at high molar ratios of aniline to formaldehyde without recycling the MDA. By the process according to the invention, minimizing the content of unwanted byproducts is possible.
The reaction according to the invention of aniline with formaldehyde, preferably in the presence of acid catalysts, is performed according to the invention semicontinuously, i.e. one reaction component, the aniline and preferably the acid catalyst, is introduced and the second reaction component, the formaldehyde with or without acid catalyst, is added to the first reaction component. Preferably, the process according to the invention is carried out in such a manner that aniline and acid catalyst are introduced and formaldehyde is added to this first reaction component. The reaction is customarily carried out at temperatures of from 20 to 150° C. Preferably, the process according to the invention is carried out in such a manner that the formaldehyde is added to the reaction mixture in the circuit, i.e. to the aniline, the acid catalyst and to formaldehyde which has possibly been previously added and reaction products, up to an amount of at least 50% of the total amount of formaldehyde to be fed, preferably up to complete addition of all of the formaldehyde, at a reaction mixture temperature in the circuit of from 20 to 75° C., preferably from 20 to 60° C., particularly preferably from 30 to 40° C.
The temperature effects the isomeric distribution of the methylenedianiline in the product. If, preferentially, 2,2′-and/or 2,4′-methylenedianiline are to be prepared, a high temperature may be advantageous. The reaction mixture can be heated by generally customary devices, e.g. by heat exchangers in the pumped circuit or a second pumped circuit and/or via the reactor wall.
The reaction mixture, after feeding into it at least 50% of the total amount of formaldehyde to be fed, is, preferably towards the end of the feed of formaldehyde solution, particularly preferably after the complete addition of the entire amount of formaldehyde to the reaction mixture, heated, preferably for a period of at least 0.2 hours, particularly preferably from 0.2 to 48 hours, in particular from 0.2 to 6 hours, at a temperature of above 75° C., preferably above 90° C., particularly preferably from 105 to 150° C., especially from 110 to 135° C. Particularly preferably, after complete addition of the formaldehyde to the reaction mixture, the reaction mixture can be heated for a period of from 0.1 to 120 minutes at a temperature of from 65 to 100° C. and then, as described above, at a temperature of above 75° C. This heating offers the advantage that the handleability of the reaction mixture is simplified, since the reaction mixture has a lower viscosity at the higher temperature. At the same time, during this heating, unwanted byproducts in the reaction mixture are broken down or rearranged in an ageing phase. The reaction mixture can be aged under these preferred conditions in the apparatus in which the reaction of formaldehyde with aniline was carried out, or else batchwise or continuously in another apparatus into which the reaction mixture can be transferred after complete addition of the formaldehyde. For example, the reaction mixture can be aged in the apparatus in which the formaldehyde solution is fed or was fed. It is also possible to pass the reaction mixture from the apparatus into at least one further reactor, for example a tubular reactor and/or stirred tank, and to perform the ageing in this reactor (these reactors) at a temperature of above 75° C. Preferably, the reaction mixture, after complete addition of the formaldehyde, is transferred to another apparatus in which the ageing is completed. Particularly referably, the reaction mixture, after complete addition of the formaldehyde which took place preferably at a temperature of from 20 to 60° C., particularly preferably from 30 to 40° C., is transferred into a customary storage vessel, heated as described preferably at a temperature of from 65 to 100° C. and then heated in conventional reactors, preferably a tubular reactor, as described preferably at a temperature of from 105 to 150° C., particularly preferably from 110 to 135° C.
The reaction mixture can thus be passed into, for example, tubular reactors, stirred tanks, stirred tank cascades, combinations of stirred tanks and tubular reactors in which the reaction to give MDA can be completed.
The reaction mixture comprising MDA and customarily polymeric MDA can be worked up after the reaction by generally known processes, for example by neutralization, phase separation, distillation and/or chromatographic separation methods, prefe
Jacobs Jan
Penzel Ulrich
Scharr Volker
Schwarz Hans Volkmar
Schweers Olaf
BASF - Aktiengesellschaft
Borrego Fernando
Cameron Mary K.
Gorr Rachel
LandOfFree
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