Distillative separation of ketazine from polyurethane dispersions

Abstract

The present invention relates to a process for removing ketazine from polyurethane dispersions by means of distillation below the boiling point of ketazine.

Claims

1. Process for the removal of acetone and a ketazine from an aqueous polyurethane dispersion comprising: providing an aqueous polyurethane dispersion containing acetone and a ketazine, wherein the ketazine is a reaction product of acetone and hydrazine; removing the acetone by distillation of the aqueous polyurethane dispersion at a selected pressure and an increasing vapour temperature until reaching a vapour temperature at which removal of the acetone is achieved; and continuing distillation at the selected pressure and an increased vapour temperature until removal of the ketazine is achieved; wherein the increased vapour temperature constantly exceeds the vapour temperature at which removal of the acetone is achieved by no more than 10%.

2. Process according to claim 1, wherein the selected pressure is at least 120 mbar and the increased vapour temperature is at most 53° C.

3. Process according to claim 2, wherein a bottom temperature during the continuing distillation is at most 53° C.

4. Process according to claim 2, wherein the removing of acetone by distillation is commenced with a bottom temperature of at most 45° C. and the continuing distillation is performed with an increased bottom temperature of up to a maximum of 53° C.

5. Process according to claim 2, wherein the selected pressure does not exceed 140 mbar.

6. Process according to claim 1, wherein the vapour temperature at which removal of the acetone is achieved is 48° C., and the continuing distillation takes place for at least two hours.

7. Process according to claim 1, wherein more than 90% of the ketazine originally present in the aqueous polyurethane dispersion is removed.

8. Process according to claim 1, wherein the polyurethane of the aqueous polyurethane dispersion was constructed by chain extension of an isocyanate-functional prepolymer with hydrazine.

9. Process according to claim 1, wherein the ketazine content of the aqueous polyurethane dispersion at the end of the continuing distillation is below 1000 ppm.

Description

EXAMPLE 1: PREPARATION OF A CRUDE DISPERSION

(1) The crude dispersion was prepared as follows: a polyester polyol and further polyols having a molar mass <400 g/mol were initially charged in a polymerization reactor and heated to 70° C. A polyisocyanate mixture was then metered in and the internal reactor temperature Increased to 100′C. The reaction mixture was stirred at 100′C until the theoretical NCO value of 4.47% by weight had been reached. The resulting isocyanate-functional prepolymer was then cooled to 60° C. and dissolved in acetone. After complete dissolution in acetone, the prepolymer solution was transferred to the distillation reactor and, at 40′C, the aqueous solution of a mixture of the sodium salt of aminoethylaminoethanesulphonic acid with hydrazine for the chain extension was added with stirring and the mixture then stirred for a further 15 min. Finally, the dispersion with water was carried out. The slightly milky polyurethane crude dispersion had a pH of 6.9 and a solids content of 23.8% by weight.

EXAMPLE 2 (COMPARATIVE): CONVENTIONAL DISTILLATION OF THE CRUDE DISPERSION

(2) The crude dispersion was heated to 40° C. in the distillation reactor. A vacuum was then applied which was lowered stepwise to 120 mbar. After reaching 120 mbar, the bottom temperature and also the vapour temperature increased continuously. On reaching a vapour temperature of 48.5° C., the acetone content of the dispersion was below 1% by weight and the distillation was terminated.

(3) The ketazine content at the end of the distillation was 1100 ppm.

EXAMPLE 3: INVENTIVE DISTILLATION PROCESS

(4) The crude dispersion was heated to 40° C. in the distillation reactor. A vacuum was then applied which was lowered stepwise to 120 mbar. After reaching 120 mbar, the bottom temperature and also the vapour temperature increased continuously.

(5) After reaching a vapour temperature of 48.5° C., the bottom temperature was increased to 51° C. over a time period of two hours at an unchanged pressure of 120 mbar.

(6) At the end of the distillation process according to the invention, the ketazine content was 65 ppm.