Method for separating isocyanate monomers

Abstract

The invention relates to a method for the separation of isocyanate monomers from isocyanate-containing mixtures by the provision of the mixture in a solvent and dialysis of the dissolved mixture against the solvent by means of a permeable membrane having a pore size in the range of between 5 and 400 nm. The method may in particular be employed for the separation of isocyanate monomers from prepolymers containing isocyanate groups, with said prepolymers being used for the production of adhesives, insulating, and expanding foams.

Claims

1. A method for separating an isocyanate monomer from a mixture comprising an isocyanate-containing prepolymer and the isocyanate monomer, the method comprising dissolving the mixture in acetone as a solvent, and subjecting the dissolved mixture to dialysis against acetone with a permeable membrane made from polyetheretherketone, wherein the membrane has a pore size in the range of between 5 and 400 nm.

2. The method according to claim 1, wherein the membrane has a pore size ranging between 15 and 200 nm.

3. The method according to claim 1, wherein the membrane has a thickness of between 0.05 and 50 μm.

4. The method according to claim 1, wherein said method is used for the separation of monomeric diphenylmethane diisocyanate (MDI), toluylene diisocyanate (TDI) or hexamethylene diisoyanate (HDI) from prepolymers.

5. The method according to claim 1, wherein the dialysis takes place in a dialysis vessel, wherein the dialysis vessel is divided by the permeable membrane, wherein one side of the membrane comprises the dissolved mixture and the other side of the membrane comprises acetone and separated monomer, and wherein during dialysis the dissolved mixture is flowed in a direction countercurrent to that of the flow of the acetone and separated monomer.

6. The method according to claim 1, wherein the concentration of the mixture in the solvent is in the range of between 2 and 20% w/w.

7. The method according to claim 6, wherein the concentration of the mixture in the solvent is in the range of between 4 and 10% w/w.

8. The method according to claim 1, wherein the dialysis is carried out between 0.1 and 4.0 h.

9. The method of claim 2, wherein the pore size is in the range of between 25 and 100 nm.

10. The method of claim 3, wherein the membrane thickness is between 0.5 and 40 μm.

11. The method of claim 3, wherein the membrane thickness is between 5 and 20 μm.

Description

EXAMPLE 1

(1) With a dialysis volume of 2 ml an MDI-containing prepolymer, dissolved in TMCP at a ratio of 1:20, was dialyzed against 1000 ml of TMCP. For dialysis, a polyethersulfone membrane having a pore size of 55 nm was used. The thickness of the membrane was 20 μm.

(2) The source material contained 5.8% w/w of monomeric MDI.

(3) After a dialysis duration of half an hour the content of monomeric MDI was found to be below the detection limit.

EXAMPLE 2

(4) A solution of a prepolymer with a content of 13.2% w/w of monomeric MDI in TMCP at a ratio of 1:20, dialysis volume 2 ml, was dialyzed against 200 ml of TMCP. After a dialysis time of ten minutes the sample content of MDI was 7.4%, after thirty minutes of dialysis it was found to be 2.9%.

(5) Comparable results were achieved with acetone, THF, and ethyl acetate being used as solvent.

EXAMPLE 3

(6) FIG. 1 shows a schematic representation of a dialysis apparatus to be used in accordance with the invention. In a dialysis vessel 1 the dissolved mixture (channel 2) and the solvent (channel 4) are run countercurrently in channels 2 and 4 separated by a permeable membrane 3. Passing through membrane 3 the isocyanate monomers in channel 2 and are entering the solvent present in channel 4 from where they are evacuated.

(7) The representation shows a discontinuous process configuration wherein the mixture is circulated (channel 2, line 5) and collected in collecting vessel V2.

(8) Depletion of the monomer takes place in several cycles through membrane 3. Countercurrently, the solvent is run through line 6 and channel 4. The monomer-laden solvent accumulates in collecting vessel V1. The solvent is separated from the monomer solution by distillation (distillation/condensation D) and added again to the solvent stream. The monomer accumulates in vessel V1.

(9) If the dialysis capacity is adequate, the method may also be carried out continuously. In this case, a single pass through the dialysis apparatus is sufficient. The separating efficiency of the apparatus is especially governed by the membrane surface, the flow velocity of the media, and the ratio between membrane surface and existing volume of the medium.