Primer for digital printing
The present invention is directed to primer coatings that enhance the adhesion of liquid digital printing inks and toners to the substrate. The coatings comprise one or more polyurethane dispersions and one or more acrylic emulsions. The coatings of the present invention improve resistance to mineral oils, such as those typically found in liquid digital printing inks and toners. The primer coatings of the invention also reduce blocking, so that a primer coated reel of substrate may be easily unrolled, even after having been stored in the rolled reel for an extended period of time.
1. A primer coating for enhancing adhesion of a digital liquid printing ink or toner to a substrate comprising:
a) 45 wt % to 85 wt % of one or more polyurethane dispersions, based on the total weight of the primer; and
b) 10 wt % to 30 wt % of one or more self-crosslinking acrylic emulsions, based on the total weight of the primer; and
wherein the glass transition temperature (Tg) of the polyurethane is less than −55° C.; and wherein at least one self-crosslinking acrylic has a Tg above 25° C.
2. The primer coating of claim 1 , further comprising one or more additional solvents.
3. The primer coating of claim 2 , wherein at least one of the one or more additional solvents is water.
4. The primer coating of claim 2 , wherein the one or more additional solvents are present in an amount of 0.1 wt % to 30 wt %, based on the total weight of the primer.
5. The primer coating of claim 1 , comprising two or more polyurethane dispersions.
6. The primer coating of claim 5 , wherein the two or more polyurethane dispersions comprise at least one polyester polyurethane and at least one polyether urethane.
7. The primer coating of claim 1 , comprising at least one polyurethane having a Tg between −80° C. and −60° C.
8. The primer coating of claim 1 , comprising at least one self-crosslinking acrylic having a Tg in the range of 25° C. to 70° C.
9. The primer coating of claim 1 , comprising at least one self-crosslinking acrylic having a Tg in the range of 30° C. to 50° C.
10. The primer coating of claim 1 , further comprising one or more additives independently selected from the group consisting of one or more antifoam additives, one or more anti-bacterial agents, and one or more colorants.
11. The primer coating of claim 10 , wherein the one or more additives are present in an amount of 0.1 wt % to 3 wt %.
12. The primer coating of claim 1 , further comprising one or more anti-blocking agents.
13. The primer coating of claim 12 , wherein the one or more anti-blocking agents are present in an amount of 0.5 wt % to 5 wt %.
14. The primer coating of claim 12 , wherein the one or more anti-blocking agents are independently selected from the group consisting of waxes, silica, metal anhydrides, and combinations thereof.
15. A printed article comprising the primer coating of claim 1 .
16. A method of priming a substrate, comprising applying the primer coating of claim 1 , and drying the primer coating.
17. A method of enhancing adhesion of a digital liquid printing toner or ink to a substrate, comprising:
a) providing a substrate;
b) applying onto the substrate the primer coating of claim 1 ;
c) drying the applied primer coating;
d) applying a digital liquid printing ink or toner onto the dried primer coating; and
e) drying the applied digital liquid ink or toner.