Method for producing light-coloured TDI-polyisocyanates
The invention relates to polyisocyanates comprising urethane groups, based on toluylene diisocyanate, to a method for their production, and to their use as the polyisocyanate component in one- and two-component polyurethane coatings.
1. A method for producing polyisocyanates comprising urethane groups and having APHA colour indices <50 Hazen comprising reacting di- to tetra-valent organic polyhydroxy compounds with excess amounts of toluylene diisocyanate and then removing unreacted excess toluylene diisocyanate to residual contents <0.5 wt. % by distillation, wherein the toluylene diisocyanate has a content of 2-chloro-6-isocyanato-methylcyclohexadienes <5 wt·ppm.
2. The method according to claim 1 , wherein the toluylene diisocyanate is produced by gas phase phosgenation.
3. The method according to claim 1 , wherein the toluylene diisocyanate has a content of 2-chloro-6-isocyanato-methylcyclohexadienes below the detection limit of 1 wt·ppm.
4. The method according to claim 1 , wherein the polyisocyanates comprising urethane groups have APHA colour indices <30 Hazen.
5. The method according to claim 1 , wherein the toluylene diisocyanate is 2,4-toluylene diisocyanate or a mixture thereof with up to 35 wt. %, based on the mixture, of 2,6-toluylene diisocyanate.
6. The method according to claim 1 , wherein the polyhydroxy compounds are di- to tetra-hydric alcohols having a molecular weight of from 62 to 146 and/or polyether polyols having a molecular weight in the range of from 106 to 600 prepared therefrom by addition of ethylene oxide and/or propylene oxide.
7. The method according to claim 1 , wherein the polyhydroxy compounds are a mixture of trimethylolpropane and diethylene glycol.
8. A polyisocyanate comprising urethane groups produced by the method according to claim 1 .
9. A method comprising utilizing the polyisocyanates comprising urethane groups according to claim 8 as the polyisocyanate component in a polyurethane coating.
10. The method according to claim 9 wherein the polyisocyanates comprising urethane groups is a crosslinker in two-component polyurethane coatings.
11. A polyurethane adhesives comprising the polyisocyanate produced by the method according to claim 8 .