METHOD AND APPARATUS FOR PRODUCING A POLYURETHANE DISPERSION HAVING REDUCED FOAM FORMATION

Abstract

The invention relates to a method for producing a polyurethane dispersion, comprising the steps: I) providing polyurethane polymers and/or polyurethane prepolymers A) in a liquid phase comprising a first solvent, which solvent is miscible with water and has a lower boiling point than water; II) in the event that isocyanate functional polymers or isocyanate functional prepolymers were provided in step I): adding NCO-reactive compounds such that, at least in part, a reaction with the polymers or prepolymers occurs; III) distilling off the first solvent, which is miscible with water, such than an aqueous polyurethane dispersion is obtained; wherein the liquid phase in step I) still comprises water and/or after step II), water is added to the mixture obtained according to step II). Foam bubbles created during step III) are contacted at least temporarily by a second solvent. The first and second solvents are preferably acetone. The invention further relates to an apparatus for carrying out the method according to the invention.

Claims

1. A process for producing a polyurethane dispersion, comprising: I) providing polyurethane polymers and/or polyurethane prepolymers A) in a liquid phase comprising a water-miscible first solvent having a lower boiling point than water; II) if isocyanate-functional polymers or isocyanate-functional prepolymers are provided in step I): adding NCO-reactive compounds to form a mixture, such that at least partial reaction with the polyurethane polymers and/or polyurethane prepolymers A) occurs; and III) distilling off the water-miscible first solvent to obtain an aqueous polyurethane dispersion, wherein foam bubbles formed during step III) are at least temporarily contacted with a second solvent, wherein the liquid phase in step I) comprises water and/or, after step II), water is added to the mixture.

2. The process as claimed in claim 1, wherein the polyurethane prepolymers A) are obtained from the reaction of A1) organic polyisocyanates with A2) monomeric polyols and/or polymeric polyols having number-average molecular weights of ≥400 g/mol to ≤8000 g/mol and OH functionalities of ≥1.5 to ≤6.

3. The process as claimed in claim 1, wherein isocyanate-reactive compounds A3) having molecular weights of 62 to 399 g/mol are added in step II).

4. The process as claimed in claim 1, comprising adding hydrophilizing agents A4) in step II), wherein the hydrophilizing agents A4) comprise cationic or potentially cationic hydrophilizing agents, anionic or potentially anionic hydrophilizing agents, nonionic hydrophilizing agents, or a combination thereof.

5. The process as claimed in claim 1, wherein the polyurethane dispersion comprises a defoamer in an amount of ≤1% by weight, based on a total weight of the polyurethane.

6. The process as claimed in claim 1, wherein the second solvent is identical to the first solvent.

7. The process as claimed in claim 1, wherein the first and second solvents are acetone.

8. The process as claimed in claim 1, comprising spraying the second solvent from one or more spray nozzles to contact the foam bubbles.

9. The process as claimed in claim 8, wherein the one or more spray nozzles are disposed in a vapor space within a vessel used for distillation.

10. The process as claimed in claim 1, comprising drawing off vapor obtained in step III) via at least one vapor tube and contacting the foam bubbles with the second solvent in the vapor tube and/or at a distance of 1 m from the vapor tube.

11. The process as claimed in claim 1, comprising applying the second solvent to the foam bubbles during step III) at a rate of ≥0.1% by volume/h to ≤20% by volume/h, based on a total volume of the dispersion present during step III).

12. The process as claimed in any of claim 1, wherein contacting the foam bubbles with the second solvent is performed exclusively within a pressure range from ≥80 mPa to ≤500 mPa.

13. The process as claimed in claim 1, wherein an amount of the second solvent applied during step III) is variable over time.

14. An apparatus for performing the process as claimed in claim 1, comprising a vessel to which a vacuum can be applied and which is set up to accommodate a polyurethane dispersion that develops foam bubbles on application of a vacuum, and including one or more nozzles through which a solvent can be applied to the foam bubbles.

15. The apparatus as claimed in claim 14, wherein the apparatus is set up to condense solvent that is distilled off and to reapply it to the foam bubbles.

Description

[0080] The present invention is elucidated in detail by the example that follows and FIG. 1, but without being limited thereto.

[0081] FIG. 1 shows an apparatus for performance of the process of the invention. This may be an appropriately modified distillation tank. The vessel 100 contains the aqueous polyurethane dispersion 200, out of which the organic solvent, for example acetone, is to be distilled. On application of a vacuum and optionally heating the dispersion 200, gaseous solvent can be drawn off at the top of the vessel 100. This is represented by stream of matter 400. In addition, foam bubbles 300 are formed on application of the vacuum.

[0082] In the vapor space, the apparatus has a nozzle 500 from which a stream 600 of a second solvent can be applied to the foam bubbles 300 in the form of individual jets or droplets 700. This results in bursting of the foam bubbles, and there is no risk of material from the dispersion 200 being entrained overhead in the apparatus.

[0083] The stream of matter 400 can be condensed and then can enter the nozzle 500 again in the form of stream of matter 600 and be applied to the foam bubbles 300.

[0084] Inventive Example: Preparation of a Polyurethaneurea Dispersion with a High Tendency to Foam

[0085] In a 500 L reactor with distillation unit, 56 kg of a polyester formed from adipic acid, hexanediol and neopentyl glycol and having an average molecular weight of 1700 g/mol and 5.5 kg of a hydrophilic monofunctional polyether based on ethylene oxide/propylene oxide (number-average molecular weight 2250 g/mol, OH number 25 mg KOH/g) were heated up to 65° C. Subsequently, 13.6 kg of isophorone diisocyanate (IPDI) was added and the mixture was stirred at 120° C. until the NCO value had gone below the theoretical value of 3.0%.

[0086] The resultant prepolymer was dissolved by adding 134 kg of acetone under pressure and simultaneously cooled down to 40° C. Subsequently, a solution of 3.2 kg of isophoronediamine (IPDA) in 24 kg of water was metered in at 40° C. The mixture was stirred for a further 15 min.

[0087] This was followed by dispersion by addition of 162 kg of water within 10 minutes. Directly thereafter, the crude dispersion was cautiously evacuated. At a pressure of 300 mbar (temperature: 34° C.), vigorous evolution of foam commenced, which made further distillation impossible.

[0088] It was only by the inventive spraying of the foam surface in the reactor by means of acetone from a flat-jet nozzle present above the liquid level (supply pressure: 3 bar, spray angle: 120°, flow rate calibrated with water: 15 kg/h, droplet size: about 200 μm) that it was possible to instantaneously break down the foam.

[0089] It was possible to efficiently continue the distillative removal of the acetone still present in the reaction mixture at this juncture by further cautious reduction of the pressure and slight raising of the internal reactor temperature. Briefly ending the inventive spraying of acetone after a distillation time of 5 hours by way of experiment led directly back to significant foam formation up to well into the vapor tube.

[0090] Therefore, the inventive spraying with acetone was restarted, which again resulted in suppression of foam formation. Even when the acetone supply pressure to the nozzle was lowered to 1.8 bar, the destruction of foam remained effective. 30 minutes before the distillation was ended, the spraying with acetone was stopped. The distillation was ended at a pressure of 150 mbar and an internal reactor temperature of 49° C. The total duration of the distillation ran to 7 hours.

[0091] A stable dispersion was obtained; the residual acetone content of the dispersion was 0.8% by weight.