Process for preparation of stable polycarbodiimide dispersions in water, which are free of organic solvents and may be used as crosslinking agent

1. A process for the preparation of stable aqueous polycarbodiimide dispersions, to be used as crosslinking agent, which are free of organic solvents, the process comprising the steps of:

reacting a polyisocyanate in the presence of a carbodiimide catalyst to form a stable polycarbodiimide;

terminating and/or chain extending the polycarbodiimide chain by the addition of a compound containing a hydrophilic group and one or more amine and/or hydroxyl functions during or after the polycarbodiimide formation; and

dispersing the resulting compound in water, wherein the pH is adjusted to a value between 9 and 14 by the addition of a base and/or a buffer to the water used for the dispersion and/or to the obtained aqueous dispersion; and

wherein said terminating or chain extending with the compound containing a hydrophilic group and one or more amine functions occurs during or after the dispersion of the polycarbodiimide in water.

2. A process according to claim 1 , wherein the pH of the polycarbodiimide dispersions is adjusted to a value between 11 and 13.

3. A process according to claim 1 , wherein said carbodiimide-forming catalyst is 1-methylphospholene-1-oxide.

4. A process according to claim 1 , in which the polyisocyanate which is used to prepare the polycarbodiimide is toluene-2,4diisocyanate, toluene-2-,6-diisocyanate and mixtures thereof, diphenylmehtane-4,4′diisocyanate, 1,4-phenylenediisocyanate, diyclohexylmethane-4,4′diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate, 1,6-hexyldiisocyanate,1,4-cyclohexyl-diisocyanate, norbonyldiisocyanate, or a mixture thereof.

5. A process according to claim 4 , wherein the polyisocyanate is dicyclohexylmehtane-4,4′-diisocyanate.

6. A process according to claim 1 wherein said base is an alkali hydroxide.

7. A process according to claim 6 wherein said base is lithium hydroxide, sodium hydroxide, potassium hydroxide, or a trialkylamine, or a trialkylamine containing hydroxyl functions.

8. A process according to claim 1 said base or part of said base is incorporated in the polycarbodiimide chain by capping the polycarbodiimide chain with a dialkylaminoalkylamine or alcohol, and/or by the addition of 0.01-0.3 equivalents regarding to the diisocynate of a tertiary amine containing polyol or polyamine during or after the polycarbodiimide formation.

9. A process according to claim 1 , wherein the effective pH range of the buffer is between 9 and 14.

10. A process according to claim 1 , wherein said compound containing a hydrophilic group and one or more pound containing a hydrophilic group and one or more amine and/or hydroxyl functions is a polyethoxy mono- or diol, a polyethoxy/polypropoxy mono- or diol, a polyethoxy mono- or diamine, a polyethoxy/polypropoxy mono- or diamine, a dial or diamine with a polyalkoxy side chain, an hydroxyl- or amine alkylsulfonate, or a dialkylaminoalkylalcohol or amine, or a mixture thereof.

11. A process according to claim 1 , wherein 0.01-0.30 equivalents of a mono- or polyol or a mono- or polyamine regarding to the polyisocyanate are added before, during or after the polycarbodiimide formation, which polyol or polyamine is for examine a mono- or polyhydroxy-alkane, a polyether mono- or polyol, a polyester polyol, a polycarbonate polyol, a polycaprolactam polyol, a mono- or polyamino-alkane, a polyether mono- or polyamine.

12. A coating mixture comprising the polycarbodimide dispersions obtained according to claim 1 as crosslinking agent and an aqueous resin containing carboxylic acid functions.

13. Cured material obtained by applying the coating mixture of claim 12 to a substrate, and evaporating the water.