Method for producing carbodiimides

Abstract

The present disclosure provides new methods for preparing carbodiimides including steps of carbodiimidizing monomeric aromatic isocyanates in the presence of a catalyst, separating low boilers and catalyst from the reaction product in a thin-film evaporator, and distilling the residue from the thin-film evaporation in a further thin-film evaporator.

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 low boilers and catalyst from the reaction product from a) in a thin-film evaporator leaving a residue comprising monomeric carbodiimides and byproducts from the carbodiimidization, and 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)
R—N═C═N—R.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 distillation step c) is performed at a temperature 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-α,α-dimethyl benzyl 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. 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

(1) CDI 1: 2,6-Diisopropylphenylcarbodiimide

(2) CDI 2: Carbodiimide of the formula (II)

(3) ##STR00004##

(4) CDI 3: Carbodiimide of the formula (I) R—N═C═N—R.sup.1, where R and R.sup.1 represent

(5) ##STR00005##

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

(7) Carbodiimidization

(8) 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-α,α-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.

(9) The carbodiimides CDI 1 to CDI 3 thus prepared were then fed into the distillations described below.

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

(11) 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.

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

(13) 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.

(14) The color was determined by the CIE L*a*b* method in accordance with ISO 11664-4. The b* value was evaluated.

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

(16) The results are shown in table 1 below.

(17) 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

(18) 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.

(19) 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.

(20) These are ideally suited for use in the production and/or stabilization of PU systems.