Process for the preparation of low-viscosity, water-dilutable urethane (meth)acrylates

1. A process for preparing a low-viscosity, water-dilutable polyurethane (meth)acrylate, comprising reacting components (a), (b), and (d)

(a) at least one organic polyisocyanate with two isocyanate functions;

(b) at least one polyoxyalkylene polyol based on a starter molecule which comprises at least three hydroxy functions and is partially reacted by esterification with (meth)acrylic acid such that on average 0.2 to 1.5 hydroxy functions still remain;

(d) at least one compound which has at least one group which is reactive towards isocyanate and at least one ionic or potentially ionic function, wherein the at least one group reactive towards isocyanate is an amino group, and wherein component (d) is an addition product of acrylic acid and a diamine, wherein the molar ratio of acrylic acid to diamine is 1:1;

to form said low-viscosity, water-dilutable polyurethane (meth)acrylate, wherein, said polyurethane (meth)acrylate contains less than 40 weight % of alkylene oxide units;

wherein

the reaction product of components (a), (b), and (d) has an NCO content of less than 0.5 weight %, and

said polyurethane (meth)acrylate is not prepared from either of

(i) organic polyisocyanates with three or more isocyanate functions, or

(ii) polyoxyalkylene polyols which have a maximum of two hydroxy functions but no further groups which can be crosslinked by actinic radiation.

2. The process of claim 1 , wherein the reacting step further comprises reacting at least one hydroxyalkyl (meth)acrylate with components (a), (b), and (d).

3. The process of claim 1 , wherein a further component (e), which is an organic compound which has one or two isocyanate-reactive functions and a molecular weight of less than 240 g/mol, is reacted with components (a), (b), and (d).

4. The process of claim 1 , wherein component (d) is at least one compound with at least one group which is reactive towards isocyanate and at least one potentially anionic group.

5. The process of claim 1 , wherein a further component (e), which is an organic compound which has one or two isocyanate-reactive functions and a molecular weight of less than 240 g/mol, is reacted with components (a), (b), and (d) and is employed in amounts of from 0 to 20 weight %, wherein the reacting step further comprises reacting 0 to 20 weight % of at least one hydroxyalkyl (meth)acrylate with components (a), (b), and (d), wherein component (a) is employed in amounts of from 8 to 40 weight %, component (b) is employed in amounts of from 40 to 90 weight %, component (d) is employed in amounts of from 1 to 6 weight % and wherein the sum of the weight % of the at least one hydroxyalkyl (meth)acrylate and components (a), (b), (d) and (e) does not exceed 100.

6. The process of claim 1 , wherein component (d) is an addition product of acrylic acid on ethylenediamine.

7. The process of claim 1 , wherein component (d) is a PUD salt.

8. The process of claim 1 , wherein component (d) is N-(2-aminoethyl)-beta-alanine.

9. The process of claim 1 , wherein the diamine is isophoronediamine.

10. The process of claim 1 , wherein the diamine is ethylenediamine.

11. The process of claim 1 , wherein the molar ratio of component (a) to component (b) is between 1:1 to 1:1.5.

12. A water-dilutable polyurethane (meth)acrylate obtained by the process of claim 1 .

13. A coating comprising the water-dilutable polyurethane (meth)acrylate of claim 12 .

14. The coating of claim 13 , wherein said coating is a lacquer or adhesive.

15. A coating composition comprising

A) at least one polyurethane (meth)acrylate of claim 12 ;

B) compounds which differ from A) and contain groups which react, by polymerization, with ethylenically unsaturated compounds under the action of actinic radiation;

C) aqueous binders which are not radiation-curable;

D) initiators;

E) optionally water and/or solvents; and

F) optionally auxiliary substances and additives.

16. A coating composition comprising

A) at least one polyurethane (meth)acrylate of claim 12 ;

B) compounds, dissolved or dispersed in water, which contain unsaturated radiation-curable groups;

C) compounds, dissolved or dispersed in water, which contain no unsaturated radiation-curable groups;

D) initiators;

E) optionally water and/or solvents; and

F) optionally auxiliary substances and additives.

17. A substrate coated with the composition of claim 15 .

18. The substrate of claim 17 , wherein said substrate is wood, ligneous substrates, cork, and substrates containing cellulose fibres.

19. A process for preparing a low-viscosity, water-dilutable polyurethane (meth)acrylate, comprising reacting components (a), (b), and (d)

(a) at least one organic polyisocyanate with two isocyanate functions;

(b) at least one polyoxyalkylene polyol based on a starter molecule which comprises at least three hydroxy functions and is partially reacted by esterification with (meth)acrylic acid such that on average 0.2 to 1.5 hydroxy functions still remain;

(d) at least one compound which has at least one group which is reactive towards isocyanate and at least one ionic or potentially ionic function, wherein the at least one group reactive towards isocyanate is an amino group, and wherein component (d) is an addition product of acrylic acid and a diamine, wherein the molar ratio of acrylic acid to diamine is 1:1;

to form said low-viscosity, water-dilutable polyurethane (meth)acrylate, wherein said polyurethane (meth)acrylate contains less than 40 weight % of alkylene oxide units;

wherein

the reaction product of components (a), (b), and (d) has an NCO content of less than 0.5 weight %, and

said polyurethane (meth)acrylate is not prepared from either of

(i) organic polyisocyanates with three or more isocyanate functions, or

(ii) polyoxyalkylene polyols which have a maximum of two hydroxy functions but no further groups which can be crosslinked by actinic radiation;

wherein the low-viscosity, water-dilutable polyurethane (meth)acrylate has a shear viscosity at 23° C. in accordance with DIN EN ISO 3219 of ≦20 Pas; and

wherein, after addition of water the viscosity of the water-dilutable polyurethane (meth) acrylate at a solids content of from 70 to 95 wt. % is the same as or lower or lower than above a solids content of 95 wt. %; and

wherein, after addition of water, the water-dilutable polyurethane (meth)acrylate forms a homogeneous phase above a solids content of 70 wt. %.