Patent ID: 8764333

Claim:
A water based ink composition for a ballpoint pen containing ι-carrageenan as an essential component of a viscous property controlling agent, in addition to a dye, a naphthalene-based dye intermediate, water and a water-soluble organic solvent, wherein a shear thinning index (n; 20° C.) of the ink composition obtained from the following experimental formula: Experimental formula T=Kj n [T: shear stress (dyne/cm 2 ), K: constant, j: shear rate (s −1 ), n: shear thinning index] is approximately 0.4 to 0.9 at a viscosity [cone and plate rotary viscometer (EL type: 20° C.)] of 1 or more and less than 20 mPa·s.