1. Field of the Invention
The present invention relates to a water-based ink composition for ballpoint pen and a water-based ballpoint pen using the composition. More specifically, the present invention pertains to a water-based ink composition for ballpoint pen which overcomes the problems such as blur and failure in writing owing to the clogging at the writing tip portion and pertains to a water-based ballpoint pen using the composition.
2. Description of the Related Art
A large number of water-based ballpoint pens having a water-based ink for ballpoint pen directly filled in an ink reservoir tube and equipped with an ink follower at the rear portion of the ink have been put on the market.
The ink composition to be filled in the ballpoint pen contains a shear-thinning tackifier so that the ink composition exhibits a high viscosity and is retained stably in a ballpoint pen mechanism when it is allowed to stand without a shear stress, while by a shear stress caused by the rotation of the ball upon writing, the ink composition has a reduced viscosity in the vicinity of the ball and is therefore transferred onto a paper surface.
Although an ink composition having a polysaccharide as the shear-thinning tackifier is conventionally disclosed, the polysaccharide, when it exists in the ink, has a particle size of about 10 μm or greater and even if it has a relatively small particle size, it exceeds 5 μm.
Ballpoint pens filled with the above-described conventional ink composition therefore do not always show a satisfactorily smooth flow. When such an ink composition is filled in a ballpoint pen equipped with a ballpoint pen tip holding a small-diameter ball and having a narrow clearance between the ball and the ball receiver portion and this ballpoint pen is put into practical use, the shear-thinning tackifier tends to cause clogging in the ballpoint pen tip and damage the smooth ink discharge, which sometimes leads to deterioration in writing performance such as blur or failure in writing (refer to, for example, Japanese Patent Unexamined Publication No. JP-A-59-74175).