Electrostatic ink compositions

1. An electrostatic ink composition comprising:

a) chargeable particles comprising a resin;

b) a charge director comprising a sulfosuccinate salt of the general formula MA m , wherein M is a metal, m is the valence of M, and A is an ion of the general formula (I):

[R 1 —O—C(O)CH 2 CH(SO 3 )C(O)—O—R 2 ] −   (I)

wherein each of R 1 and R 2 is an alkyl group; and

c) a non-polymeric, non-fatty acid having a pK a , when measured in water at 20° C., of at least 1.

2. The electrostatic ink composition according to claim 1 , wherein the non-polymeric, non-fatty acid is selected from carboxylic acids, phenolic acids, amino acids, and optionally substituted uric acids.

3. The electrostatic ink composition according to claim 1 , wherein the non-polymeric, non-fatty acid is a hydroxyacid.

4. The electrostatic ink composition according to claim 3 , wherein the hydroxyacid is a substituted alkyl having at least one substituent that is a carboxyl group and at least one substituent that is a hydroxyl group, wherein the alkyl has from 2 to 10 carbons.

5. The electrostatic ink composition according to claim 1 , wherein the non-polymeric, non-fatty acid is a polyacid.

6. The electrostatic ink composition according to claim 5 , wherein the polyacid is of the formula HO 2 C-L-CO 2 H, wherein L is a single bond or an organic linker group selected from an optionally substituted aliphatic moiety and an optionally substituted aromatic moiety.

7. The electrostatic ink composition according to claim 1 , wherein the non-polymeric, non-fatty acid is selected from citric acid, oxalic acid, succinic acid, lactic acid, uric acid, arginine and formic acid.

8. The electrostatic ink composition according to claim 1 , wherein the non-polymeric, non-fatty acid has a pK a , when measured in water at 20° C., of from 3 to 6.

9. The electrostatic ink composition according to claim 1 , wherein the non-polymeric, non-fatty acid constitutes from 0.5 wt % to 4 wt % of the solids of the electrostatic ink composition.

10. The electrostatic ink composition according to claim 1 , wherein the composition further includes a liquid carrier and the chargeable particles comprising the resin are dispersed in the liquid carrier.

11. A method of producing an electrostatic ink composition, the method involving:

producing chargeable particles comprising a resin;

and, before, during or after the chargeable particles comprising the resin are produced, combining the resin with

(i) a charge director comprising a sulfosuccinate salt of the general formula MA m , wherein M is a metal, m is the valence of M, and A is an ion of the general formula (I):

[R 1 —O—C(O)CH 2 CH(SO 3 )C(O)—O—R 2 ] −   (I)

wherein each of R 1 and R 2 is an alkyl group, and

(ii) a non-polymeric, non-fatty acid having a pK a , when measured in water at 20° C., of at least 1.

12. The method according to claim 11 , wherein a liquid carrier, the resin and the non-polymeric, non-fatty acid are ground together before the charge director is added.

13. The method according to claim 12 , wherein the resin is ground for a period of time, and the non-polymeric, non-fatty acid is added to the resin being ground during the last 50% of the period of time.

14. The method according to claim 12 , wherein the resin is ground for a period of time, and the non-polymeric, non-fatty acid is added to the resin being ground during the first 50% of the period of time.

15. A print substrate having printed thereon an electrostatic ink composition comprising:

a) a resin;

b) a charge director comprising a sulfosuccinate salt of the general formula MA m , wherein M is a metal, m is the valence of M, and A is an ion of the general formula (I):

[R 1 —O—C(O)CH 2 CH(SO 3 )C(O)—O—R 2 ] −   (I)

wherein each of R 1 and R 2 is an alkyl group; and

c) a non-polymeric, non-fatty acid having a pK a , when measured in water at 20° C., of at least 1.