Aqueous polyurethane/polyurea dispersions
The present invention pertains to aqueous polyurethane-polyurea dispersions which are prepared by the acetone process using methyl acetate instead of ketones and are chain-extended using hydrazine or derivatives thereof, these dispersions having an improved stability towards thermal yellowing and a relatively low residual hydrazine/hydrazine derivative content.
1. Process for preparing aqueous polyurethane-polyurea dispersions which are stable to thermal yellowing, comprising the steps of reacting polyisocyanates, polymeric polyols or polymeric polyamines, or both, and isocyanate-reactive hydrophilicizing compounds to form an NCO-containing prepolymer, and conducting a chain extension reaction with hydrazine or hydrazine hydrate, or both, wherein
A) the NCO-containing polyurethane prepolymer is prepared by reacting
A1) polyisocyanates selected from the group consisting of aliphatic isocyanates and araliphatic isocyanates with
A2) polymeric polyols, polyamines or both, wherein the polyols, polyamines or both have number-average molecular weights of 400 to 8000 g/mol,
A3) optionally, low molecular weight polyalcohols, polyamines or amino alcohols, having number-average molecular weights of 17 to 400 g/mol, and
A4) isocyanate-reactive, ionically, potentially ionically, nonionically hydrophilicizing compounds or a mixture thereof,
A6) in methyl acetate,
B) optionally the prepolymer obtained from step A is diluted by further addition of a solvent, A6), and
C) the remaining free NCO groups of the prepolymer are reacted with hydrazine or hydrazine hydrate as chain extenders so as to attain an arithmetic degree of chain extension of 40% to 100%, and
wherein water is added by introducing the dissolved prepolymer or the chain extended polyurethane polymer into the dispersing water, or the dispersing water is added to the prepolymer or the polymer solutions.
2. Process according to claim 1 , wherein the chain extenders are hydrazine hydrates or an aqueous hydrazine solution.
3. An aqueous polyurethane-polyurea dispersion stable towards thermal yellowing, produced in accordance with a process comprising the steps of reacting polyisocyanates, polymeric polyols or polymeric polyamines, or both, and isocyanate-reactive hydrophilicizing compounds to form an NCO-containing prepolymer, and conducting a chain extension reaction with hydrazine or hydrazine hydrate, or both, wherein
A) the NCO-containing polyurethane prepolymer is prepared by reacting
A1) polyisocyanates selected from the group consisting of aliphatic isocyanates and araliphatic isocyanates with
A2) polymeric polyols, polyamines or both, wherein the polyols, polyamines or both have number-average molecular weights of 400 to 8000 g/mol,
A3) optionally, low molecular weight polyalcohols, polyamines or amino alcohols, having number-average molecular weights of 17 to 400 g/mol, and
A4) isocyanate-reactive, ionically, potentially ionically, nonionically hydrophilicizing compounds or a mixture thereof,
A6) in methyl acetate,
B) optionally the prepolymer obtained from step A is diluted by further addition of a solvent A6), and
C) the remaining free NCO groups of the prepolymer are reacted with hydrazine or hydrazine hydrate as chain extenders so as to attain an arithmetic degree of chain extension of 40% to 100%, and
wherein water is added by introducing the dissolved prepolymer or the chain extended polyurethane polymer into the dispersing water, or the dispersing water is added to the prepolymer or the polymer solutions.
4. A coating, adhesive bond, sealant or moulding produced from an aqueous polyurethane-polyurea dispersion of claim 3 .
5. A substrate coated with a coating of claim 4 .
6. A process according to claim 1 , wherein the arithmetic degree of chain extension is of 60% to 100%.
7. A process according to claim 1 , wherein the arithmetic degree of chain extension is of 85% to 100%.
8. A process according to claim 1 , wherein plasticizers or solvents which boil above 100° C., which are non-ketonic and which are inert or only very slightly reactive towards isocyanate and hydrazine are added to the reactants in the preparation of the prepolymer, and wherein A2 comprises polymeric polyols selected from the group consisting of polycarbonate diols, polyester diols, including lactone-based polyester diols, and polyether diols.
9. An aqueous polyurethane-polyurea dispersion according to claim 3 , wherein plasticizers or solvents which boil above 100° C., which are non-ketonic and which are inert or only very slightly reactive towards isocyanate and hydrazine are added to the reactants in the preparation of the prepolymer, and wherein the dispersion comprises no detectable amount of hydrazine when using colorimeter determination with 4-dimethylamino-benzaldehyde at 1000 times dilution of the dispersion.