DE1950101A1 - Process for the purification of diphenyl methane-4,4'-diisocyanate - Google Patents

Description

FARBENFABRIKEN BAYER AG ^1950101

LE VERKU S £ N - Beyerwerk GM / Tr | Patent department

'3. OKI. 1969

Process for the purification of Diphenylmethan- 4 , 4 '-diisoeyanat

Diphenylmethane-4,4'-diisocyanate is an industrially manufactured ^v notified Diisoeyanat, the wide range of applications, in particular for the production of Polyurethanelastoineren, polyurethane elastomer fibers and polyurethane synthetic leather place. It is produced by phosgenation of diphenylmethane-4,4'-diamine or technical mixtures of this diamine which contain more highly condensed compounds and which are preferably obtained by condensation of formaldehyde and aniline in the presence of strong acids such as hydrochloric acid. From the crude phosgenation mixtures, which can be used directly, for example, for the production of rigid polyurethane foams, a fraction which predominantly contains diphenylmethane-4,4'-diisoeyanate is then distilled out and optionally re | distilled. The accompanying substance occurs in smaller quantities, for example between 0.5 and 5 #> the 2.4 ^t ~ isomer.

Solid diphenyl methane-4,4'-diisoeyanate obtained in this way shows a tendency to a troublesome undesirable yellowing when stored within a relatively short time, the few hours or days. A number of measures have already been proposed to avoid this yellow discoloration e.g., distillation of diphenylmethane-4,4'-diisoeyanate over certain metals, e.g. zinc, iron, brass, etc., or a distillation over certain metal salts, e.g.

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PeGl ,. These processes are intended to destroy or convert impurities that cause the discoloration; but it was So far not possible, a diphenylmethane-4,4'-diisocyanate that remains permanently colorless manufacture in this way. Also, by adding "stabilizers" of any kind, e.g. of phenolic antioxidants, Aethylenimiη derivatives or !!, N-dialkylsemicarbazide derivatives, attempts have been made to stain the G-elbe to prevent diphenylmethane-4i4'-diisocyanate. All of these measures were only moderately successful, as they were apparently the real cause of the yellow discoloration could not be eliminated. In addition, some of these additives have an effect on the activity of the diisocyanates into the polyurethanes produced with the aid of 4,4'-biisocyanate with introduced, where they may cause undesirable side effects.

It was found _s now a process, these öelbverfärbung of diphenylmethane ~ 4 "4'-diisocyanate without additives to eliminate such that even a re-coloring of the diisocyanate is in storage completely prevented.

It is described in DAS 1 543 258, from crude mixtures of substances containing di- and polyisocyanates by selective extraction with two inert, relatively non-polar solvents to isolate a liquid polyisocyanate in a two-phase extraction system after • separating the two phases. The procedure is also used for the Eohpliosgenation mixtures of 4 »4'-diphenylmethane diamines, but the liquid diisocyanates accessible in this way have color numbers in the best case higher than 5 on the Gardener scale. Ss will also be liquid diisocyanate mixtures) obtained with discarding a very high proportion of diisoeyanate crude mixture. On the other hand-

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The process according to the invention relates to the complete elimination of yellow discolouration from solid diphenylmethane-4,4'-diisocyanate with recovery of solid, practically isomer-free (about <0.5 # foreign isomers, preferably <0.2 $) diphenylmethane-4,4'- '-diisocyanate, which _(is practically colorless and, above all, remains colorless during further storage. (Gardener color number <3, preferably e <2).

The present invention relates to a process for the purification of diphenylmethane-4,4'-diisocyanate using inert organic solvents, characterized in that diphenylmethane-4,4'-diisocyanate is prepared, if appropriate with the action of catalysts, until the formation of an intensely yellow colored body and then dissolves the yellow colored body of separated from the solution, and optionally recrystallized from the solution. A preferred mode of operation consists in storing the diphenylmethane-4,4'-diisocyanate after preparation for a period of at least one day, optionally with the action of catalysts, at from 0 to 80 ^° C. until an intensely yellow-colored, sparingly soluble body has formed and that the solvent used is one with a dielectric constant below 2.3. A process is preferred which consists in using solvent mixtures of inert, nonpolar or slightly polar, optionally halogenated aliphatic solvents and aromatic hydrocarbons and / or aliphatic monoethers, preferably of the formula ROR (R ^ g-Cg-alkyl or cycloalkyl radicals) .

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The method is based on the finding that when the solid diphenylmethane-4,4'-diisocyanate is stored, the yellow discoloration of the Diisocyanates does not cause the general discoloration of the substance, which is usual for organic substances, but that this occurs Yellow coloring is caused by the inclusion of defined, yellow, solid, relatively high-melting particles of an intensely yellow reaction product. These yellow particles are .in low polar, inert organic solvents, preferably with a dielectric constant of less than 4.5 »preferred less than 2.3, e.g. in boiling petroleum ether or in hot petroleum fractions, insoluble and can thus be surprisingly easily and completely by filtration or centrifugation removed. While such a "solution" of aged, i.e. H. yellowed diphenylmethane-4,4'-diisocyanate in E.g. petrol appears yellow and slightly cloudy before filtration, the filtrate is completely colorless and the residue on your filter is intense yellow. That from the filtrate Diphenylmethane-4,4'-diisocyanate that crystallizes out is completely colorless and remains even when stored in a glass bottle even without complete exclusion of air for very long periods of time, e.g. several years, colorless, for example, old samples are after a storage time of 3 years, remained colorless,

If, on the other hand, you use freshly distilled diphenylniethan-4,4'-diisocyanate, which can be completely colorless from the distillation and which only turns yellow after storage, directly recrystallized from petroether under the same conditions, this initially gives colorless diphenylmethane-4,4'-diisocyanate, which, however, turns yellow again on storage. the Causes causing the yellow discoloration are thus not or only partially removed during recrystallization, if you have a previous storage time of the diphenylmethane-4,4'-diisocyanate waived.

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A final stabilization of the diphenylmethane-4,4 ^t -diisocyanate is, as has been found, only achieved by first allowing all the compounds present which induce the yellow discolouration to react to form the insoluble corpus luteum and only then allowing them to react in the form of those in the Solvents to be used insoluble yellow particles are completely removed by filtration. These inducing compounds could not be removed using the previous purification processes.

The chemical nature of the yellow particles has not yet been clarified and is certainly complex. Analyzes and spectra indicate a noticeably enriched content of diphenylmethane-2,4'-diisocyanate and a certain content of bonded sulfur and chlorine * traces of water, oxygen, ^v hydrogen halides or CarbaminsäurecHoriden or carboxylic acid chlorides, preferably in amounts less than T by weight, practice has a catalytic effect on the rate of formation of the corpus luteum. Infrared spectroscopic findings also indicate a considerable content of uretdione groups in the corpus luteum, which result from the dimerization of isocyanate groups. It is conceivable that the inducing compounds attach to these uretdione-containing polymeric diisocyanates or react with them particularly easily and relatively firmly. The chemical cause of the formation of the corpus luteum in diphenylmethane-4 »4'-diisocyanate has therefore not yet been clarified. In addition to a titratable isocyanate content of about 20-25 $> { NCO, according to IR spectra, these yellow bodies also contain small but significant amounts of urea groups, which may result from the reaction of the diisocyanates with traces of water. Intensive analytical investigations indicate that the yellowing is evidently not caused by a uniform chemical compound, but by a group of substances with a chemically similar constitution, including groups of the type R-CH = NR.

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The "aging" of the diphenylmethane-4 »4'-diisocyanate is in itself unproblematic. It is achieved by allowing the preferably crystalline, flaky material to stand for a period of at least one day, preferably more than three days. As storage temperatures are O as appropriate to 80 ⁰ C, especially 0 to 6O ° C, preferably allowed to stand at room temperature. At higher temperatures, the dimerization rate and possibly carbodiimide formation increases undesirably. The yellow discoloration is accelerated by traces of moisture and air, such as those found in the technical manufacture of flaky goods. Even with the action of very small amounts of hydrogen chloride (for example from phosgenation residues), a noticeably accelerated yellowing also occurs, which can also be caused by other substances listed above. A cleaning operation according to the method according to the invention is expediently carried out when the yellow discoloration has evidently occurred and is no longer (significantly) increasing. The yellow discoloration can also be done quantitatively by measuring the absorption at 427 m μ in a benzene solution (or suspension of the molten diisocyanate in benzene). In the case of "aging", the access of oxygen seems to be necessary in order to cause the yellow coloration.

The nature and the surprising effect of the process according to the invention become clear when one considers the unsuccessful dissolving of unaged diphenylmethane-4,4'-diisocyanate from the solvents to be used according to the invention and the effect of others for the recrystallization of diphenylmethane-4 _s 4 '-diisocyanate also assessed in principle as a useful solvent.

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It can easily be determined in experiments that even from very yellow, aged diphenylmethane-4,4'-diisocyanate, which have large amounts of corpus luteum, when dissolved in highly polar solvents a clear, residue-free, yellow Solution is obtained which is filtered without changing color. From the yellow filtrate, yellow and continued to crystallize again discoloring diisocyanate. Examples of such highly polar solvents which are not suitable according to the invention are their effect is indicated in the following table.

TABLE I.

Solvent dielectric appearance filter filtrate

Solder residues of constant quality (E / ° C) solution

Dimethyl form
amide > 20 clear, gel
be solution none yellow lo
sung Dimethylacetate
amide > 20 It Il Il Acetonitrile > 2O Il Il Il acetone 20.7 / 20 ° Il ti Il Methyl ethyl -
ketone 18.5 / 20 ° Il Il Il Formamide 109/20 ° H η Il Glycol formal. - Il Il H chloroform 4.81 / 20 ° Il Il ti Methylene
chloride 9.08 / 20 ° Il Il Il

Another group of solvents also solves this Diphenylmethane-4,4'-diisocyanate and those contained in it Yellow bodies on, with even a clear solution and a noticeable lightening of the color of the solution or a colorless solution can be obtained (see Table H):

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TABLE II

Solution Dissolve Dielectric Appearance Filter Filtrate medium temperature conditional solution return

constant (E / ° C) stand

Dioxane

Ethyl acetate

40 ⁰ C

2.21

40-45 ⁰ C 6.0 / 25 °

M; Tetrahydrofuran 40 g

clear, color- none colorless-yellow- (xx) yellowish borrowed (x)

clear, almost none almost colorless colorless

clear, almost none almost colorless colorless

x) When the solution is heated to approximately boiling temperature occurs an intense yellow-brown discoloration.

xx) If aged diisocyanates are used, which larger quantities to uretdione groups from the dimerization reaction of Contain isocyanate groups, these remain polymeric Dimerization products as a colorless residue.

When the diphenylmethane-4,4'-diisocyanate crystallizes out again from these solvents, however, it is found that the The corpus luteum and its causes have not been sufficiently removed and that the diisocyanate is either still yellow or turns yellow again in a short time.

A number of other less polar aromatic solvents without polar substituents (e.g. methoxy, nitro groups) such as chlorobenzene, toluene or benzene, dissolve the diphenylmethane-4,4'-diisocyanate at room temperature or a little higher temperature, however, they also dissolve the majority of the poorly soluble corpus luteum. Diiso-

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cyanate is not colorless and color stable. Diethyl ether behaves similarly, as does the above aromatic solvents - can only be used according to the invention. Melted diphenylmethane-4,4'-diisocyanate is also in accordance with the invention not suitable as a solvent. Part of the corpus luteum remains undissolved and can therefore be removed by filtration, whereby the residue of the melt appears lighter However, a further part remains dissolved, so that the desired, completely colorless, color-stable diphenylmethane-4,4'-diisocyanate is not obtained is obtained.

In contrast, it has been shown that when using less polar, inert, aliphatic solvents or solvent mixtures, preferably of a lipid nature and a low dielectric constant (Dk <4.5, preferably <2.3), the aged, yellowed diphenylmethane-4, 4'-diisocyanate pure diisocyanate detach easily can un the yellow, hoehschmelzende body in these solvents remains unsolved, if you are not too high, for example, better not choose the solution temperature, about 15O ⁰ C 125 ⁰ C. Preferably, one solves the diphenylmethane-4,4'-diisocyanate in the aliphatic solvents, preferably at temperatures below 80 ⁰ C, for example at 40 - 60 ⁰ C out, in order to avoid in any case that even small proportions of the corpus luteum 'in Solution go. In principle, it is also possible to work with solvents at room temperature, but this is inexpedient in the case of the crystallization of the solid diphenylmethane-4,4'-diisocyanate from these solvents. 4 ¹ -diisocyanate, for example in cyclohexane or mineral spirits, to work for the production of polyurethanes without isolating the diisocyanate in solid form beforehand. -

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The relation between the amount of solvent and the aged diisocyanate one expediently chosen so that a good separation of the sparingly soluble corpus luteum is possible, e.g. one uses the equal to ten times the amount, preferably 2 to 6 times the amount by weight of solvents, based on diisocyanate.

The suitability of an inert, little polar aliphatic solvent or solvent mixture for obtaining pure, colorless diphenylmethane-4,4 ¹ -

diisocyanates can be determined in a simple preliminary test ~ ■ will be. To this end, the aged, yellow diphenylinethane is dissolved .4 »4'-diisocyanate in 4 to 6 times the amount of solvent at room temperature and at higher! temperatures. The typical corpus luteum impurities must be in undissolved form Remaining shape (at the same time, if applicable, remain existing polymeric (colorless) dimerization products undissolved) and the filtrate must be colorless. Not suitable In the context of the invention, solvents are those which dissolve the corpuscles of the yellow and give a yellow filtrate or else 'if they show the corpus luteum in a more or less, lightened, or also contain colorless solution form. The maximal Dissolving temperature for the mis ooyanat is limited by the fact that possibly Parts of the corpus luteum go into solution or melt or cool down again.

Solvents suitable for the purposes of the invention are more particularly Slightly polar, inert, ubiquitous solvents, e.g. aliphatic Hydrocarbons with dielectric constants below 2.3 e.g. hexane, heptane, octane, dodecane, cyelohexane, Methylcylohexane, cyclooctane, cyclododecane or hydrocarbon mixtures with predominantly aliphatic components, as they are called petroleum ether, petrol, petroleum ether or ligroin is technically available as distillates with certain boiling limits. Preferred as to be used according to the invention

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Solvents are aliphatic G ₅ - Cp _n hydrocarbons and / or perchlorethylene. It is also preferred that aliphatic Cc - Cg ^ hydrocarbons and / or perchlorethylene are used in an amount from 2 "to 10 times the amount by weight," based on 4,4'-diphenylinethane diisocyanate, temperatures of about 0 to 80 ^° C. during dissolution be respected. Solvents with a low dielectric constant such as carbon tetrachloride and especially perchlorethylene have proven to be particularly suitable solvents. And aromatic hydrocarbons (eg benzene, toluene, xylene, decalin, chlorobenzene) or aliphatic monoethers such as Dicyclohexylather or diethyl ether can be found partly use when i their dissolving power to the luteal characterized reduced, that they, with at least equal parts by weight of aliphatic hydrocarbons preferably mixed with twice to 10 times the amount. For example, a mixture of 1 part benzene and 5 parts methylcyclohexane is a suitable solvent. Likewise, araliphatic hydrocarbons in which the aliphatic radical predominates with its number of carbon atoms, for example dodecylbenzene or octylbenzene, are suitable solvents for the diisocyanate without dissolving the corpus luteum.

The diphenylmethane-4> 4'-diisocyanate, which after filtration of the yellow impurity crystallizes out of the solutions, is colorless and remains even when stored with the ingress of I Traces of oxygen and moisture (e.g. in a normally closed Glass bottle) colorless for months and years. It has also been found that in this cleaning operation the foreign isomer content is reduced very significantly, so that only very small proportions of 2,4'-isomers being found. These 2,4'-isomers are likely to accumulate very strongly in the corpuscles of the luteum and thus become removed by filtration of this gel body. Further be

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in this cleaning operation also the polymeric dimerization products eliminated that have formed during storage. The solid, practically isomer-free diphenylmethane-4,4'-diisocyanate then has a very high freezing point.

The yields of pure diphenylraethane-4,4'-diisocyanate are very high and can be increased by using the mother liquor several times for the cleaning operation. It is, however, because of a certain accumulation of the 2; 4'-isomers In the mother liquors, it is advisable to work up the mother liquors after one or more cycles and with fresh w solvent to work. The process can also be operated continuously by having a continuous extraction according to the Soxletz principle, or e.g. by means of boiling Petroleum ether, makes. The residues of the yellow body or from the mother liquors can be fully utilized by adding them, for example, to the bohphosgenation products of the diphenylmethane bases which then e.g. for the production of rigid foams can be used.

The thus purified, colorless, practically isomer-free dipbonylmethane-4,4'-diisocyanate can be used with preference in sensitive diisocyanate polyaddition reactions, e.g. for the production of polyurethane elastomer spinning solutions, for thermoplastic polyurethane elastomers or, if necessary, for microporous polyurethane coatings. It shows a more uniform reactivity towards OH-containing reactants, facilitates the conduct of the reaction and provides better color stability of the polyurethanes produced with it. Compared to isomer-containing diphenylmethane-4,4'-diisocyanate, the melting point of the process product is also increased, or the "heat distortion temperature" of polyurethanes produced therefrom is improved.
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Examples Example 1

500 parts of a technically pure I) iphenylmothane-4,4'-diisocyanate (produced by distillation of the crude phosgenation product of 4,4'-diamino-diphenylmethane), which was almost colorless immediately after production and was clear after half a day After a storage time of 22 days in a glass flat at room temperature (after which it had assumed an intense yellow color which, according to visual assessment, was no longer significantly increased) with 3000 parts of petroleum ether, it showed a tendency towards increasing yellow discoloration to about 60-65 G heated and the cloudy, yellow solution filtered. On filtration, an intensely yellow, powdery filter residue remains in an amount of 6.4 parts (1.28 percent by weight of the diisocyanate used). After cooling with ice, 440 parts of colorless di, phenylmethane-4,4 ^l -diisocyanate crystallize from the completely clear and colorless filtrate. The colorless diisocyanate remains colorless for a period of 3 years when stored in a glass surface in a place protected from direct sunlight (the test is terminated after this time). In the case of direct exposure to solar radiation on a south-facing window for 8 weeks, the diisocyanate remains practically colorless (the test is terminated after this time). The purified diisocyanate does not show more yellow, \ typical absorption, contains substantially less 2,4'-isomer and other impurities such as sulfur. The analyzes of the starting material, the yellow residue and the pure, colorless diisocyanate are listed in the table.

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TABEL

Initial inaxerial (after 22 ~ days Storage)

yellow Hüeksxand

colorless diisocyanate from filtrate

Color ■

Solubility (petroleum ether)

NCO content (esp. on pure diisocyanate)

2,4'-isomer content

Sulfur content (ppm)

Abs orp t i onBmaxiimini (Benad.)

Melting point. - Survey

yellow

cloudy, yellow "solution"

99.85 ⁰ A

1.5 1 *

low uretdione content ("Dirnere")

Xe_A12_552 intense yellow

insoluble, bsw ^ very difficult to dissolve

68.5 %

19.0

ppm

colorless

Easily soluble, colorless solution

99.95

0.06

<10 ppm

427 mn

115 to 122 ⁰ C

additionally su stronger Uretdione band (in

1770 cm * -) and stronger NCO bands kick a lot strong new bands at 1380 and 1400, strong shoulder at 154.0, 1520 on 151 O-band, weak new bands at 1710 1720, 1120, 1045 »762,

491. The Urea Volumes

at 1640 cm "^ - is noticeably reinforced

no dimer, spectrum

pure

After 3 years of storage, the colorless Diisoeyanat noel · contains about 97.5 $ of the theory of NOO groups, but something (colorless) dimerization has formed, which remains ⁰ C unresolved in Petifcäther or benzene at 4O be (colorless) and separated can. After this long storage time, the retdione groups of the dimerization product can also be detected in the IR spectrum.

Example 2

250 parts of a yellow, aged (2 months at room temperature) diphenylmethane-4,4'-diisocyanate at 55 - 60 ⁰ C and treated 15OO parts Petrdläther high-boiling and the hot, yellow solution is cloudy filtered with suction. An intensely yellow residue remains on the suction filter (4.7 parts = $ 1.88 of the weight used). The FiItrat appear completely colorless and clear and, after cooling in ice and filtering off the precipitated colorless diisocyanate, and again, concentrating the mother liquor a total of 232.1 parts (97.0 $ of the diisocyanate used to colorless, color-stable Diphenylmetlian-4, A-diisocyanate If the mother liquor is concentrated even further, about 1 g of diisocyanate is still obtained The properties and isomer contents are shown in the table below.

IiCO according to 2.4 '-! So- color number IR-spec- appearance

Source material

Titration

yellow residue 64.5 % of theory

purified 99.6
Diisocyanate

meres 0.7 56

3.2 %

η. Gardner tren

12th

colorless

something dimer

contains

much

Dimers

yellow

intense yellow

free from colorless diine ren

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Remaining diisocyanate from mother liquor: almost colorless crystals, more enriched in 2,4'-isomers. '

Example 3

100 parts aged, solid diphenylmethane-4,4'-diisocyanate (see. Example 2) are heated to 5O ⁰ C perchlorethylene minutes with 400 parts and filtered nocht hot. .It a yellow residue remains (7 parts by weight) unsolved containing much polymeric Diiaerisationsprodukt and from about 120 ⁰ C at about 180 ° clear melts after sintering. The NCO content is $ 59 »6 > d. Th. (Calculated on pure diisocyanate). After concentration, 84 parts of colorless and color-stable diisocyanact (Gardener number <Ί.7), NOO content of 99.6% of theory, isomer content of 0.1% of > 2,4'-isomer, are obtained from the colorless piltrate. The diisocyanate has a freezing point of 39 ⁰ C ■

If diisocyanate that has aged for a short time is treated (see example 1), which contains fewer dimerization products than above, so with similar treatment 1.9 parts become insoluble The corpus luteum is filtered off and 94 parts from the colorless filtrate colorless, highly purified diphenylmethane-4,4'-diisocyanate isolated. .

Example 4

100 parts aged, yellow solid diphenylmethane-4,4'-diisocyanate having a 2,4 'isomer content of from about O ₉ 6? 6 are after 6 months of storage at room temperature with 600 parts Cyclohexan15 minutes at 50 ⁰ C. and then the warm solution tears away. A yellow color remains

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Residue (6 parts) which, according to the IR spectrum, has a very high dimer content. The isocyanate content is 63.4 i "theory. The substance begins to sinter at 11O ⁰ C and melted at 150 ⁰ C to smaller residues. Completely colorless diisocyanates with 2,4-isomer content be less than 0.1 and # obtain a solidification point of 39.3 ⁰ C, indicating an excellent purity of the substance from the Eiltrat after concentration to 1/3 82 parts. The diisocyanate remained completely colorless within an observation period of 4 weeks when stored in a glass bottle and without exclusion of oxygen.

Example 5

Solid, aged diphenylmethane-4,4'-diisocyanate (8 days Aging time) is treated with the following solvents to remove the corpus luteum:

Solvent tempera-corpus luteum volatility

door residue

An isolated diphenylmethane-4,4'-diisocyanate ¹

Methyl-cyclo-100 ° hexane

Ether / perchlorethylene (10: $ 0) approx. 38 °

Dicyclohexyl ether / cyclohexane (10:50) 55

Benzene / detergent. _ petrol (10:80) 60 °

Toluene / detergent _ petrol (10/90) 70 °

Perchlorethylene

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1.85 % 1.9 # 1.85 % 1.90

- 17 -

colorless colorless, stable

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Solvent Tempera- Tannin Filtrate Isolated

door residue Diphenylmetfaan-

4.4 ^f -diisocyanate

Carbon tetrachloride / perchlorethylene (20:80) 55 Dodecylbenzene 70 °

1.90
1.90

colorless colorless, stable

Example 6

35 parts of the older diisocyanate used in Example 5 are filled into an extraction thimble and extracted with high-boiling petroleum ether for several hours. By the The pure diphenylmethane-4,4'-diisocyanate was extracted from the solvent dripping back from the cooler. As a residue 1.5 parts of the yellow, insoluble compound remain. The extract becomes colorless, color-stable diisocyanate with a freezing point of 39 »15 ° G is obtained.

Example 7

As in Example 5, 35 parts of the diisocyanate are extracted from the sleeve using perchlorethylene. After 2 1/1 hours of fe, a yellow, insoluble residue (1 part) remained in the thimble. Mach concentration of the perchlorethylene colorless diphenylmethane-4,4'-diisocyanate crystallized from ^r of high solidification point (39,3O ⁰ C). The diisocyanate is color-stable when stored at room temperature. According to the thin-layer chromatogram, the diisocyanate is free from isomers.

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Claims (1)

Claims : ßQ / i. / Method for the purification of diphenylmethane-4,4'-Qiisocyanat using inert organic solvents, characterized in that diphenylmethane-4,4'-diisocyanate ¹ after the preparation, if appropriate under the action of catalysts, until a deep yellow zur.Ausbildung colored sparingly soluble body and then using inert, non-polar or slightly polar, optionally halogenated aliphatic solvents with a dielectric constant below 4.5 at temperatures of 0 to 125 ⁰ C dissolves, separates the yellow-colored body from the solution, and optionally from the Solution recrystallized. 2. The method according to claim 1, characterized in that diphenylmethane-4,4'-diisocyanate after preparation for a time of at least one day at 0 to 80 ⁰ C, optionally under the action of catalysts, until an intensely yellow colored poorly soluble Body stores and then using inert, · non-polar or slightly polar, optionally halogenated ali-. Dissolves phatic solvents with a dielectric constant below 2.3 at temperatures from 0 to 125 ° C, separates the yellow-colored body from the solution and, if necessary, recrystallized from the solution. 5. The method according to claim 1, characterized in, that the solvent is mixtures of inert, non-polar or slightly polar, optionally halogenated aliphatic Solvents and aromatic hydrocarbons and / or aliphatic monoethers of the formula ROR (R = C ₂ -C ₆ -AlkVl- or cycloalkyl radicals) is used. Le A 12 532 - 19 - ■ 1 0 9 B ι ^ / ν.: 2 1 4. The method according to claim 1, characterized in that the solvent used is aliphatic C ₁ - - C ^ "hydrocarbons and / or perchlorethylene. 5. The method according to claim 1, characterized in that aliphatic Cc - Cjp hydrocarbons are used in a 2- to 10-fold amount by weight, based on diphenylmethane-4,4'-diisocyanate, at temperatures of 0 to 80 ⁰ C for dissolving . Le A 12 532 - - 20 - 1 0 9 8 1 5/2 2 2 1- ¹ ORJQINALiNSPECTiD