Electrostatic ink compositions

1. An electrostatic ink composition, wherein the composition is formable by combining:

chargeable particles comprising a resin comprising a polymer having acidic side groups which are free of ester groups, and further comprising a polymer having ester side groups, and

a cyclic anhydride, wherein the cyclic anhydride is benzophenone tetracarboxylic dianhydride.

2. The electrostatic ink composition according to claim 1 , wherein the cyclic anhydride constitutes from 0.5 wt % to 3 wt % of the solids of the electrostatic ink composition.

3. The electrostatic ink composition according to claim 1 , wherein the electrostatic ink composition further comprises a charge director.

4. The electrostatic ink composition according to claim 3 , wherein the charge director comprises a sulfosuccinate moiety of the general formula [R 1′ —O—C(O)CH 2 CH(SO 3 − )C(O)—O—R 2′ ], where each of R 1 ′ and R 2 ′ is an alkyl group.

5. The electrostatic ink composition according to claim 4 , wherein each of R 1′ and R 2′ independently is a C 6-25 alkyl.

6. The electrostatic ink composition according to claim 1 , wherein the polymer having acidic side groups is a co-polymer of (ii) a first monomer having acidic side groups selected from acrylic or methacrylic acid and (iii) a second monomer which is an alkylene monomer selected from ethylene and propylene.

7. The electrostatic ink composition according to claim 1 , wherein the composition further comprises a carrier liquid and the chargeable particles comprising the resin comprising the polymer having acidic side groups are dispersed in the liquid carrier.

8. A method of forming the electrostatic ink composition of claim 1 , the method comprising combining the chargeable particles and the cyclic anhydride.

9. A method of electrostatic printing, the method comprising:

providing an electrostatic ink composition according to claim 1 ,

contacting the electrostatic ink composition with a latent electrostatic image on a surface to create a developed image,

transferring the developed image to a print substrate.

10. The electrostatic ink composition according to claim 1 , wherein the polymer having ester side groups is a co-polymer of a first monomer having ester side groups, a second monomer having acidic side groups, and a third monomer which is an alkylene monomer.

11. The electrostatic ink composition according to claim 10 , wherein the first monomer constitutes 8% to 12% by weight of the co-polymer, the second monomer constitutes 8% to 12% by weight of the co-polymer, and the third monomer constitutes the remaining weight of the co-polymer.