Aqueous barrier layer based on polyurethane dispersions

A process for preparing a polyurethane dispersion comprising forming an isocyanate- and acid-functional polyurethane by reacting

• A) 55 to 75 wt. % of a mixture of 20 to 80 wt. % of an aliphatic polyester diol having a number average molecular weight of 840 to 2100 and 80 to 20 wt. % of an aliphatic polycarbonate diol or polyester carbonate diol having a number average molecular weight of 1000 to 2100,
• B) 1 to 3 wt. % of a trifunctional, low molecular weight alcohol,
• C) 20 to 35 wt. % of isophorone diisocyanate and/or hexamethylene diisocyanate, or a mixture of 4,4′-diisocyanatodicyclohexyl-methane with isophorone diisocyanate or hexamethylene diisocyanate and
• D) 3.5 to 4.9 wt. % of dimethylol propionic acid,
• adding a neutralizing agent in an amount sufficient to neutralize 40 to 105% of the acid groups and dispersing the prepolymer in water,
• chain extending said prepolymer in water at 25 to 75° C., and
• removing organic solvent during or after formation of the dispersion or during or after the chain extension reaction so that the amount of organic solvent comprises less than 5%, based on the weight of the aqueous dispersion.

A process for preparing a polyurethane dispersion comprising forming an isocyanate- and acid-functional polyurethane by reacting

• A) 55 to 75 wt. % of a mixture of 20 to 80 wt. % of an aliphatic polyester diol having a number average molecular weight of 840 to 2100 and 80 to 20 wt. % of an aliphatic polycarbonate diol or polyester carbonate diol having a number average molecular weight of 1000 to 2100,
• B) 1 to 3 wt. % of a trifunctional, low molecular weight alcohol,
• C) 20 to 35 wt. % of isophorone diisocyanate and/or hexamethylene diisocyanate, or a mixture of 4,4′-diisocyanatodicyclohexyl-methane with isophorone diisocyanate or hexamethylene diisocyanate and
• D) 3.5 to 4.9 wt. % of dimethylol propionic acid, in the presence of a neutralizing agent in an amount sufficient to neutralize 40 to 105% of the acid groups with
• dispersing the resulting prepolymer in water,
• chain extending said prepolymer in water at 25° C. to 75° C., and
• removing organic solvent during or after formation of the dispersion or during or after the chain extension reaction so that the amount of organic solvent comprises less than 5%, based on the weight of the aqueous dispersion.

A coating composition comprising

• a) 30 to 90 wt. % of a polyurethane dispersion wherein the polyurethane comprises the reaction product of
• A) 55 to 75 wt. % of a mixture of 20 to 80 wt. % of an aliphatic polyester diol having a number average molecular weight of 840 to 2100 and 80 to 20 wt. % of an aliphatic polycarbonate diol or polyester carbonate diol having a number average molecular weight of 1000 to 2100,
• B) 1 to 3 wt. % of a trifunctional, low molecular weight alcohol,
• C) 20 to 35 wt. % of isophorone diisocyanate and/or hexamethylene diisocyanate, or a mixture of 4,4′-diisocyanatodicyclohexyl-methane with isophorone diisocyanate or hexamethylene diisocyanate and
• D) 3.5 to 4.9 wt. % of dimethylol propionic acid,
• wherein a neutralizing agent is added in an amount sufficient to neutralize 40 to 105% of the acid groups and an amount sufficient to neutralize at least 40% of the acid groups is added prior to the chain extension reaction, wherein at least 1 wt. % of components A) and B), based on the total weight of components A) to D), are tri- or higher-functional compounds and wherein the polyurethane contains 1 to 4 wt. % of the urea groups set forth in brackets in the following formula:
• b) up to 20 wt. % of a crosslinking agent,
• c) 5 to 70 wt. % of a pigment and/or filler,
• d) up to 65 wt. % of a binder other than a).