METHOD FOR PRODUCING CARBODIIMIDES

Abstract

The invention relates to a novel method for preparing carbodiimides.

Claims

1. A method for preparing monomeric aromatic carbodiimides, the method comprising: a) carbodiimidizing monomeric aromatic isocyanates in the presence of a catalyst, b) separating the low boilers and catalyst from the reaction product from a) in a thin-film evaporator, c) distilling the residue from b) in a further thin-film evaporator.

2. The method as claimed in claim 1, wherein the monomeric aromatic carbodiimides are carbodiimides of the formula (I)
RNCNR.sup.1(I), where R and R.sup.1 are chosen from ##STR00006## R.sup.2=isopropenyl, t-butyl or C.sub.1-C.sub.6 alkoxy, R.sup.3=H, methyl, ethyl, isopropyl, n-propyl or tert-butyl, and A* represents the linkage to the nitrogen of the carbodiimide function in formula (I).

3. The method as claimed in claim 2, wherein the carbodiimide of the formula (I) corresponds to ##STR00007##

4. The method as claimed in claim 1, wherein the distillation steps b) and c) are operated as a continuous process.

5. The method as claimed in claim 1, wherein the distillation in step b) is carried out at a temperature of 150 C. to 220 C. and at a pressure of 0.1 to 5 mbar.

6. The method as claimed in claim 1, wherein in step b) C.sub.1-C.sub.12 alkyl-substituted benzenes, dibenzenes and/or pyrrolidones are used as entraining agents in the removal of low boilers.

7. The method as claimed in claim 1, wherein, the temperature in step c) is 5 C. higher or 10 C. higher than in step b).

8. The method as claimed in claim 1, wherein the distillation in step c) is carried out at a temperature of 160 C. to 220 C. and at a pressure of 0.05 to 5 mbar.

9. The method as claimed in claim 1, wherein the isocyanate used is 3-isopropenyl-,-dimethylbenzyl isocyanate (TMI) and/or 2,4,6-triisopropylphenyl isocyanate (TRIPI).

10. The method as claimed in claim 1, wherein the thin-film evaporator used in step c) is a short-path evaporator.

11. (canceled)

12. A method of producing a hydrolysis-stable polyurethane-based system, comprising adding to the system monomeric aromatic carbodiimides prepared according to the method of claim 1.

Description

WORKING EXAMPLES

[0049] CDI 1: 2,6-Diisopropylphenylcarbodiimide

[0050] CDI 2: Carbodiimide of the formula (II)

##STR00004##

[0051] CDI 3: Carbodiimide of the formula (I) RNCNR.sup.1, where R and R.sup.1 represent

##STR00005##

[0052] and R.sup.3 is an isopropyl radical and where A* represents the linkage to the nitrogen of the carbodiimide function in formula (I).

Carbodiimidization

[0053] The carbodiimides CDI 1 to 3 were prepared in a stainless steel tank through the reaction of the corresponding isocyanates, i.e. 2,6-diisopropylphenyl isocyanate (DIPPI) was used for CDI 1, 3-isopropenyl-,-dimethylbenzyl isocyanate (TMI) for CDI 2, and 2,4,6-triisopropylphenyl isocyanate (TRIPI) for CDI 3, in the presence of 200-500 ppm methylphospholene oxide as catalyst at temperatures of 160-170 C. with elimination of CO.sub.2. The carbodiimidization reaction was continued until a residual isocyanate content of <1% was reached.

[0054] The carbodiimides CDI 1 to CDI 3 thus prepared were then fed into the distillations described below.

a) Distillation and distillation in a thin-film evaporator (comparison) according to EP-A-0 602 477:

[0055] The carbodiimides CDI 1 to 3 were first separated from the low boilers in a batchwise process at approx. 200-220 C. and a pressure of 0.3-0.4 mbar using a Vigreux column and then continuously distilled in a VTA thin-film evaporator at temperatures of 190-200 C. and a pressure of 0.4 mbar.

b) Distillation using 2 thin-film evaporators (inventive):

[0056] The carbodiimides CDI 1 to 3 were first freed of low boilers in a VTA thin-film evaporator at temperatures of 160-170 C. and a pressure of 1.0 mbar and the residue was distilled in a second thin-film evaporator identical to the first at temperatures of 200-205 C. and a pressure of 0.4 mbar.

[0057] The color was determined by the CIE L*a*b* method in accordance with ISO 11664-4. The b* value was evaluated.

[0058] The residual content of monomeric isocyanate was determined by HPLC and the phosphorus content by X-ray fluorescence analysis (XRF).

[0059] The results are shown in table 1 below.

TABLE-US-00001 TABLE 1 Distillation + distillation in a thin-film evaporator (comp.) 2 thin-film evaporators (inv.) Content of Phosphorus Content of Phosphorus Color, monomeric content Color, monomeric content CDI b* isocyanate (%) (ppm) b* isocyanate (%) (ppm) CDI 1 10-12 >0.1 4-5 2-3 >0.1 3 CDI 2 5-6 >0.1 10-12 0-1 <0.1 <1** CDI 3 30-40 >0.1 5-6 8-9 <0.1 <1** comp. = comparison inv. = inventive **Below the detection limit of 1 ppm

[0060] The carbodiimides CDI 1 to CDI 3 prepared by the method of the invention have improved properties compared to those prepared by the prior art method.

[0061] As can be seen from table 1, the preferred carbodiimides CDI 2 and CDI 3 in particular can be prepared in particularly high quality, i.e. low color index, low content of toxic monomeric isocyanate and, moreover, having no detectable content of organophosphorus compounds.

[0062] These are ideally suited for use in the production and/or stabilization of PU systems.