Distillation of polyisocyanates
The invention relates to a method for producing polyisocyanates having a low monomer content, said method comprising the following steps: (i) modifying at least one monomeric diisocyanate to obtain a mixture containing at least one polyisocyanate and unconverted monomeric diisocyanate, (ii) separating the mixture obtained in step (i) into at least one gaseous stream containing monomeric diisocyanate and a liquid stream depleted of monomeric diisocyanate, (iii) partially condensing the gaseous stream from (ii) in at least one condenser, so that a liquid condensate and an uncondensed vapour stream are obtained, (iv) post-condensing the uncondensed vapour stream obtained in step (iii) in at least one post-condenser, so that a post-condensate and an uncondensed waste gas are obtained, and (v) delivering the uncondensed waste gas from step (iv) to the suction side of a vacuum pump, characterised in that the at least one post-condenser in step (iv) is operated at a post-condenser temperature, and the at least one condenser in step (iii) is operated at a condenser temperature, wherein the post-condenser temperature is lower by ≥1 to ≤168 K than the condenser temperature.
1 . A process for producing polyisocyanates, comprising the following steps:
(i) modifying at least one monomeric diisocyanate to obtain a mixture comprising at least one polyisocyanate and unreacted monomeric diisocyanate,
(ii) separating the mixture obtained in step (i) into at least one gaseous stream containing monomeric diisocyanate and a liquid stream depleted in monomeric diisocyanate,
(iii) partially condensing the gaseous stream from (ii) in at least one condenser, affording a liquid condensate and an uncondensed vapor stream, wherein the partially condensing in step (iii) takes place at a condenser temperature within a range from 16 to 135° C.,
(iv) performing a secondary condensation of the uncondensed vapor stream obtained in step (iii) in at least one secondary condenser, affording a secondary condensate and an uncondensed offgas, wherein the secondary condensation in step (iv) takes place at a secondary condenser temperature within a range from −33 to 130° C., and
(v) feeding the uncondensed offgas from step (iv) to the suction side of a vacuum pump, wherein the at least one secondary condenser in step (iv) is operated at a secondary condenser temperature and the at least one condenser in step (iii) is operated at a condenser temperature wherein the secondary condenser temperature is 5 to 50 K lower than the condenser temperature.
2 . The process as claimed in claim 1 , wherein the monomeric diisocyanate is selected from the group consisting of 1,5-diisocyanatopentane (PDI), 1,6-diisocyanatohexane (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI), 4,4′-diisocyanatodicyclohexylmethane, 2,4′-diisocyanatodicyclohexylmethane, bis(isocyanatomethyl)norbornane, 1,3- and 1,4-bis(isocyanatomethyl)benzene (XDI), tolylene diisocyanate (TDI), 2,4′- and 4,4′-diisocyanatodiphenylmethane (MDI), any mixtures thereof.
3 . The process as claimed in claim 1 , wherein the modifying in step (i) is an intermolecular reaction between isocyanate groups, optionally in the presence of a basic catalyst, with the formation of one or more of uretdione polyisocyanates, isocyanurate polyisocyanates and iminooxadiazinedione polyisocyanates, and/or a reaction of isocyanates with previously formed urethane groups with the formation of allophanate polyisocyanates.
4 . The process as claimed in claim 1 , wherein the liquid stream depleted in monomeric diisocyanate obtained in step (ii) contains not more than 0.5 times the content, of the monomeric diisocyanate, based on the content in the mixture obtained in step (i).
5 . The process as claimed in claim 1 , wherein the liquid condensate from step (iii) is at least partially recycled, optionally after further purification steps, to step (i) of the process.
6 . The process as claimed in claim 1 , wherein the separating in step (ii) is performed by at least one evaporator selected from the group consisting of a falling-film evaporator, a thin-film evaporator and a short-path evaporator.
7 . The process as claimed in claim 1 , wherein the separating in step (ii) is carried out as a multistage separation in 2 to 5 stages.
8 . The process as claimed in claim 6 , wherein a short-path evaporator is the final evaporator.
9 . The process as claimed in claim 1 , wherein a step (iv-a) is carried out between step (iv) and step (v), in which the uncondensed offgas stream from the secondary condenser is cooled further in a cold trap having a cold trap temperature that is at least 1 K, below the secondary condenser temperature.