Aqueous ink durability deposited on substrate

1. A method for improving durability of aqueous ink deposited on a substrate having multivalent metal salt present at a surface of the substrate, the method comprising, in order:

depositing an aqueous ink on the surface of the substrate, wherein the aqueous ink is an anionically stabilized pigment ink;

drying the substrate having the deposited aqueous ink to substantially remove the water from the deposited aqueous ink;

applying substantially pure water and heat to the dried substrate having the deposited aqueous ink; and

removing the applied water and heat to return the dried substrate having the deposited aqueous ink to an ambient temperature and moisture content, the applied moisture and heat being sufficient to cause a significant improvement of the durability of the deposited aqueous ink on the surface of the dried substrate.

2. The method of claim 1 , wherein the applying step includes providing a layer of substantially pure liquid water over the dried substrate having the deposited aqueous ink and simultaneously or subsequently re-drying the substrate.

3. The method of claim 2 , wherein the layer of substantially pure liquid water applied at a wet coverage of between 0.05 and 40 g/m 2 .

4. The method of claim 1 , wherein the applying step further includes transporting the dried substrate having the deposited aqueous ink into a chamber which is adapted to provide the substantially pure water and the heat to the dried substrate having the deposited aqueous ink.

5. The method of claim 4 , wherein the chamber is an oven and the substantially pure water is applied to the dried substrate having the deposited aqueous ink by increasing the relative humidity inside the oven.

6. The method of claim 4 , wherein the chamber is in line with the aqueous ink depositing and drying steps.

7. The method of claim 6 , wherein the substantially pure water is applied in the chamber in the form of steam across the dried substrate having the deposited aqueous ink.

8. The method of claim 1 , wherein the anionically stabilized pigment ink includes a pigment dispersant comprising an anionic surfactant or an anionic polymer.

9. The method of claim 1 , wherein the anionically stabilized pigment ink includes a self-dispersed pigment with anionic surface charge.

10. The method of claim 8 , wherein the pigment dispersant is an anionic polymer comprising carboxylate functionality.

11. The method of claim 1 , wherein the anionically stabilized pigment ink further includes a polyurethane additive.

12. The method of claim 1 , wherein the multivalent salt includes a cation selected from Mg +2 , Ca +2 , Ba +2 , Zn +2 , and Al +3 .

13. The method of claim 1 , wherein the multivalent salt includes calcium chloride, calcium acetate, calcium nitrate, magnesium chloride, magnesium acetate, magnesium nitrate, or hydrated versions of these salts.

14. The method of claim 1 , wherein the substrate includes paper, film, or composites.

15. The method of claim 1 , wherein the applied heat is in the range of 20° C. to 100° C.

16. The method of claim 1 , wherein the applied substantially pure water is the form of vapor in the range of 50% to 100% relative humidity.

17. The method of claim 1 , further comprising, subsequently to applying the substantially pure water and heat to the dried substrate having the deposited aqueous ink, passing the dried substrate having the deposited aqueous ink through a pair of rollers at elevated pressures and optionally elevated temperatures.

18. The method of claim 1 , further comprising, subsequently to applying the substantially pure water and heat to the dried substrate having the deposited aqueous ink, applying an electric field across the dried substrate having the deposited aqueous ink sufficient to drive the multivalent metal ions away from the surface of the dried substrate having the deposited aqueous ink.