The present invention pertains to an aqueous inkjet ink comprising anionic self-dispersing pigment with a certain mixture of tetraorganoammonium and alkali metal cations. The inks exhibit greatly extended latency.
1. An ink-jet ink comprising an aqueous vehicle, a colorant, and a first and second cationic species wherein:
i) said colorant comprises self-dispersed pigment with anionic dispersibility-imparting surface groups;
ii) said first cationic species is a member or any combination of members selected from the group consisting of tetrabutylammonium, benzyltriethylammmonium, butyltrimethylammonium cations, and has a molar concentration per unit weight of ink of M 1 ;
iii) said second cationic species is any member or any combination of members selected from the group consisting of Na + , K + , Rb + Cs + , and has a molar concentration per unit weight of ink of M 2 ; and
iv) the molar ratio of first cationic species to total cation concentration (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=M 1 +M 2 ).
2. The ink of claim 1 further comprising a first humectant selected from any one or combination of members of the group consisting of ethylene glycol, diethylene glycol and triethylene glycol.
3. The ink of claim 2 further comprising a second humectant selected from any one or combination of members of the group consisting of 2-pyrrolidone, di-(2-hydroxyethyl)-5,5-dimethylhydantoin, sulfolane, tetramethylene, 1,3-dimethyl-2-imidazolidinone and imidazolidinone.
4. The ink of claim 1 wherein said anionic dispersibility surface groups on the self-dispersed pigment are predominately carboxyl groups.
5. The inkjet ink of claim 1 wherein said anionic dispersibility surface groups on the self-dispersed pigment have a molar concentration per unit weight of ink of M_anionic, and Mtot is equal to or greater than M_anionic.
6. The ink-jet ink of claim 1 further comprising an anionic polymer additive.
7. The ink according to claim 6 wherein the anionic polymer additive is an anionic acrylic, styrene-acrylic or polyurethane polymer additive.
8. The ink of claim 1 wherein said colorant consists essentially of self-dispersed pigment with anionic dispersibility-imparting surface groups.
9. The ink-jet ink of claim 1 wherein said second cationic species is predominately K + .
10. The ink according to claim 9 wherein said first cationic species is predominately tertabutylammonium cations.
11. The ink of claim 1 wherein the molar ratio of first cationic species to total cation concentration (Mtot) satisfies equation 2 as follows:
0.02 ≦M 1/ M tot≦0.2 (eq. 2).
12. The ink of any of the preceding claims wherein the pigment is self-dispersed carbon black.