Method for producing methylene-diphenylene-diisocyanates and polymethylene-polyphenylene-polyisocyanates

A process for preparing methylene diphenylene diisocyanate and optionally a mixture of methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate, comprising:

• α) providing a fraction comprising methylene diphenylene diisocyanate and secondary components with a proportion by mass of methylene diphenylene diisocyanate, based on total mass of the fraction, of more than 98.0% when determined by gas chromatography, and
• β) purifying, comprising an isomer separation, the fraction comprising methylene diphenylene diisocyanate and secondary components by distillation and/or crystallization in two or more partial steps to obtain at least (i) two or more pure methylene diphenylene diisocyanate fractions, each having a proportion by mass of methylene diphenylene diisocyanate, based on their total mass, of 99.9% or more when determined by gas chromatography and (ii) a secondary component fraction having a proportion by mass, based on their total mass, of methylene diphenylene diisocyanate of 20.0% to 98.0% when determined by gas chromatography,
• wherein step α) optionally comprises:
• α.1) separating methylene diphenylene diisocyanate and secondary components from a fraction comprising methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate to obtain
• (i) a polymethylene polyphenylene polyisocyanate-enriched mixture of methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate and
• (ii) the fraction comprising methylene diphenylene diisocyanate and secondary components with a proportion by mass of methylene diphenylene diisocyanate, based on total mass of the fraction, of more than 98.0% when determined by gas chromatography;
• and
• wherein the secondary component fraction obtained in step β) is returned to:
• (1) one of the partial steps of step β) in which none of the pure methylene diphenylene diisocyanate fractions from step β) is obtained in the form of distillate or crystallizate,
• (2) step α.1), provided step α.1) is conducted, or
• (3) both:
• (a) one of the partial steps of step β) in which none of the pure methylene diphenylene diisocyanate fractions from step β) is obtained in the form of distillate or crystallizate, and
• (b) step α.1), provided step α.1) is conducted.