The present invention pertains to an aqueous inkjet ink comprising anionic self-dispersing pigment, a certain mixture of alkali metal cations and a certain humectant. The inks exhibit greatly extended latency.
1. An inkjet ink comprising an aqueous vehicle, a colorant, and a first and second cationic species wherein:
i) said aqueous vehicle comprises water and a first humectant wherein said first humectant is any member or combination of members selected from the group consisting of 2-pyrrolidone, di-(2-hydroxyethyl)-5,5-dimethyl hydantoin, sulfolane, tetramethylene sulfoxide, 1,3-dimethyl-2-imidazolidinone and imidazolidinone;
ii) said colorant comprises self-dispersed pigment with anionic carboxylate dispersibility-imparting surface groups;
iii) said first cationic species is Li + , and has a molar concentration per unit weight of ink of M1;
iv) said second cationic species is any member or combination of members selected from the group consisting of Na + , K + , Rb + and Cs + , and has a molar concentration per unit weight of ink of M2; and
v) the molar ratio of M1 to Mtot, satisfies Equation 1 as follows:
0.01≦ M 1/ M tot≦0.5 (Eq. 1)
wherein Mtot is the cumulative molar concentration of first and second cationic species per unit weight of ink (Mtot=M1+M2).
2. The ink of claim 1 further comprising a second humectant selected from any member or combinations of member of the group consisting of ethylene glycol, diethylene glycol and triethylene glycol.
3. The inkjet ink of claim 1 wherein the molar concentration of said anionic dispersibility-imparting surface groups on the self dispersing pigment per unit weight of ink is M_anionic and Mtot is equal to or greater than said M_anionic.
4. An ink according to claim 3 wherein the second cationic species is predominately K + .
5. An ink according to claim 4 wherein the molar ratio of M1 to Mtot, satisfies Equation 2 as follows:
0.01≦ M 1/ M tot≦0.15 (Eq. 2)
6. An ink according to claim 3 further comprising an anionic polymer additive.
7. The ink claim 6 wherein the anionic polymer additive is an anionic acrylic, styrene-acrylic or polyurethane polymer additive.
8. The ink of claim 7 wherein the molar ratio of M1 to Mtot, satisfies Equation 3 as follows:
0.2≦ M 1/ M tot≦0.50 (Eq. 3)
9. An ink according to claim 3 further comprising ammonium (NH 4 + ) cation.
10. An ink according to claim 3 wherein the second cationic species is predominately Na + and the molar ratio of M1 to Mtot satisfies Equation 4 as follows:
0.18≦ M 1/ M tot≦0.5 (Eq. 4)
11. An ink according to claim 3 wherein the first humectant is present in an amount of between about 2 weight % and about 30 weight % based on the total weight of ink.
12. An ink according to claim 3 wherein the first humectant is present in an amount of between about 4 weight % and about 25 weight % based on the total weight of ink.
13. An ink according to claim 3 wherein the colorant consists essentially of self-dispersed pigment with anionic dispersibility-imparting surface groups.
14. The ink of any of the preceding claims wherein the pigment is self-dispersed carbon black.
15. An ink according to claim 3 wherein the decap time of said ink is substantially longer when compared to a ink of similar composition, but with no first cationic species; the decap time being defined as the amount of time an inkjet printhead, loaded with an ink, can be left uncapped and idle and still fire a drop of that ink without misdirection, loss of color or unacceptable decrease of velocity.