The invention relates to a method for recovering a diisocyanate that is solid at room temperature from a distillation residue originating from a production process of the diisocyanate, comprising the following steps: (i) mixing the distillation residue with at least one polyisocyanate on the basis of one or more diisocyanates different from the diisocyanate that is solid at room temperature, in such a way that a mixture is obtained that contains 70 to 90 wt. % of the distillation residue and 10 to 30 wt. % of the at least one polyisocyanate, each relative to the mixture, (ii) subjecting the mixture to distillation in a thin-film evaporator and/or a downflow evaporator, thereby obtaining a sump discharge and a gaseous product stream, and (iii) condensing the gaseous product stream and obtaining a solid containing the diisocyanate that is solid at room temperature, the at least one polyisocyanate in step (i) having a residual monomer content of ≤3.0 wt. % as determined by gas chromatography with an internal standard according to EN ISO 10283:2007-11.

The invention relates to a method for recovering a diisocyanate that is solid at room temperature from a distillation residue originating from a production process of the diisocyanate, comprising the following steps: (i) mixing the distillation residue with at least one polyisocyanate on the basis of one or more diisocyanates different from the diisocyanate that is solid at room temperature, in such a way that a mixture is obtained that contains 70 to 90 wt. % of the distillation residue and 10 to 30 wt. % of the at least one polyisocyanate, each relative to the mixture, (ii) subjecting the mixture to distillation in a thin-film evaporator and/or a downflow evaporator, thereby obtaining a sump discharge and a gaseous product stream, and (iii) condensing the gaseous product stream and obtaining a solid containing the diisocyanate that is solid at room temperature, the at least one polyisocyanate in step (i) having a residual monomer content of ≤3.0 wt. % as determined by gas chromatography with an internal standard according to EN ISO 10283:2007-11.

The invention relates to a method for recovering a diisocyanate that is solid at room temperature from a distillation residue originating from a production process of the diisocyanate, comprising the following steps: (i) mixing the distillation residue with at least one polyisocyanate on the basis of one or more diisocyanates different from the diisocyanate that is solid at room temperature, in such a way that a mixture is obtained that contains 70 to 90 wt. % of the distillation residue and 10 to 30 wt. % of the at least one polyisocyanate, each relative to the mixture, (ii) subjecting the mixture to distillation in a thin-film evaporator and/or a downflow evaporator, thereby obtaining a sump discharge and a gaseous product stream, and (iii) condensing the gaseous product stream and obtaining a solid containing the diisocyanate that is solid at room temperature, the at least one polyisocyanate in step (i) having a residual monomer content of ≤3.0 wt. % as determined by gas chromatography with an internal standard according to EN ISO 10283:2007-11.

A method for producing a composition comprising polycarbodiimide, comprising the step of the reaction of a reaction mixture which contains an aromatic polyisocyanate and a carbodiimidising catalyst, wherein, before the reaction, the aromatic polyisocyanate is treated at a temperature of ≧80° C. to ≦150° C. and a pressure of ≧1 mbar to ≦500 mbar by passing through an inert gas and/or during the reaction, the reaction mixture is treated at a temperature of ≧80° C. to ≦150° C. and a pressure of ≧1 mbar to ≦500 mbar by passing through an inert gas and wherein the content of hydrolysable chlorine in the reaction mixture is ≦10 ppm.

According to an embodiment, the pH of a diisocyanate composition and a diamine hydrochloride composition used in the preparation of an optical lens is adjusted to a specific range, whereby it is possible to enhance not only the yield and purity of the diisocyanate composition but also the optical characteristics of the final optical lens by suppressing the striae and cloudiness. Specifically, according to the process of the embodiment, the amount of an aqueous hydrochloric acid solution introduced to the reaction may be adjusted to control the pH of the diisocyanate composition to a desired range, thereby enhancing the yield and purity. Accordingly, the process for preparing a diisocyanate composition according to the embodiment can be applied to the preparation of a plastic optical lens of high quality.

According to an embodiment, the pH of a diisocyanate composition and a diamine hydrochloride composition used in the preparation of an optical lens is adjusted to a specific range, whereby it is possible to enhance not only the yield and purity of the diisocyanate composition but also the optical characteristics of the final optical lens by suppressing the striae and cloudiness. Specifically, according to the process of the embodiment, the amount of an aqueous hydrochloric acid solution introduced to the reaction may be adjusted to control the pH of the diisocyanate composition to a desired range, thereby enhancing the yield and purity. Accordingly, the process for preparing a diisocyanate composition according to the embodiment can be applied to the preparation of a plastic optical lens of high quality.

The present invention relates to a method for operating a plant for continuous production of an isocyanate by converting a primary amine A with phosgene P whilst maintaining, based on the amino groups of the primary amine, a stoichiometric excess of phosgene in the presence of a solvent L in the liquid phase, using a first, adiabatically operated reaction chamber and a second, isothermally operated reaction chamber. The method is characterised in that a combination of measures, in particular the maintenance of a sufficiently high starting pressure and a sufficiently high starting temperature, is applied in order to bring the plant back into the target state, standard operation, starting from a state of interruption to production.

The present invention relates to a method for operating a plant for continuous production of an isocyanate by converting a primary amine A with phosgene P whilst maintaining, based on the amino groups of the primary amine, a stoichiometric excess of phosgene in the presence of a solvent L in the liquid phase, using a first, adiabatically operated reaction chamber and a second, isothermally operated reaction chamber. The method is characterised in that a combination of measures, in particular the maintenance of a sufficiently high starting pressure and a sufficiently high starting temperature, is applied in order to bring the plant back into the target state, standard operation, starting from a state of interruption to production.

In the embodiments, an aqueous hydrochloric acid solution and an organic solvent instead of hydrogen chloride gas and solid triphosgene instead of phosgene gas may be used in the process of preparing a diisocyanate from a diamine through a diamine hydrochloride. In addition, the embodiments provide processes for preparing a diisocyanate composition and an optical lens, which are excellent in yield and quality with mitigated environmental problems by controlling the total content of metals, cations, or anions in a diamine hydrochloride composition.

In the embodiments, an aqueous hydrochloric acid solution and an organic solvent instead of hydrogen chloride gas and solid triphosgene instead of phosgene gas may be used in the process of preparing a diisocyanate from a diamine through a diamine hydrochloride. In addition, the embodiments provide processes for preparing a diisocyanate composition and an optical lens, which are excellent in yield and quality with mitigated environmental problems by controlling the total content of metals, cations, or anions in a diamine hydrochloride composition.