Circulation process for the production of aliphatic and cycloaliphatic diisocyanates

A process for producing a (cyclo)aliphatic diisocyanate of the formula

• OCN--R.sup.1 --NCO
• comprising the steps of:
• converting a (cyclo)aliphatic diamine of the formula
• H.sub.2 N--R'--NH.sub.2
• to a biscarbamate of the formula
• R.sup.2 --O--CO--NH--R.sup.1 --NH--CO--OR.sup.2
• and thermally cracking said biscarbamate, to obtain said diisocyanate, an alcohol of the formula R.sup.2 OH and by-products, which comprises at least one compound selected from the group consisting of high molecular weight, undistillable compounds containing uretdione, isocyanurate, allophanate, urea, polyuret, or carbodiimide groups;
• wherein:
• (i) said (cyclo)aliphatic diamine and said by-products are reacted with urea and said alcohol of the formula R.sup.2 OH in the presence of N-unsubstituted carbamates and dialkyl carbonates with the simultaneous separation of any ammonia formed, to obtain a reaction mixture containing said biscarbamate, unreacted alcohol, N-unsubstituted carbamate, and dialkyl carbonate;
• (ii) said biscarbamate obtained in (i) is separated from said unreacted alcohol, N-unsubstituted carbamate, and dialkyl carbonate, which are recycled into (i) and from any unutilizable product;
• (iii) said cracking of said biscarbamate is carried out continuously in the liquid phase, without solvent, in the presence of a catalyst, with the reaction mixture boiling, and with fractionation of the vapors which contain said diisocyanate and said alcohol of the formula R.sup.2 --OH;
• (iv) said diisocyanate and said alcohol obtained in said cracking are fractionally condensed as crude products;
• (v) a portion of the cracking reaction mixture containing said by-products is continuously discharged and recycled to said reaction in (i) after reacting with said crude alcohol from (iv);
• wherein:
• R.sup.1 is a straight-chain or branched aliphatic hydrocarbon group with a total of 4 to 12 carbon atoms or a substituted or unsubstituted cycloaliphatic hydrocarbon group with a total of 5 to 13 carbon atoms, and
• R.sup.2 is a group obtained by removing the hydroxyl group from a primary aliphatic alcohol with 1 to 8 carbon atoms.