Method of making and the composition for an initially erasable ink for a ball point writing instrument

The method of making an ink composition which is characterized by its initial erasability by ordinary pencil erasers when applied by a ball-point writing instrument to an absorbent paper-like writing surface and which thereafter develops permanence, said method comprising the steps of:

• (a) selecting elastomeric material from the group consisting of natural rubbers, synthetic rubbers and mixtures thereof, said elastomeric material having a low level of hard resinous gel particles and an extremely low level of retained dirt which is no more than 0.05% by weight;
• (b) masticating said selected elastomeric material by subjecting same to high shear stresses within the range of about 2.0.times.10.sup.6 to 2.0.times.10.sup.7 KG. Meter sec .sup.-2 (Newtons) to soften said elastomeric material to a Mooney viscosity within the range of about 17 to 27;
• (c) coloring said masticated elastomeric material by introducing colored pigment into said masticated elastomeric material while subjecting same to further shear stresses so as to forcibly impregnate said material with pigment, said color pigment being selected from the group consisting of dry pigment, flushed pigment and a paste consisting of pigment dispersed in a rubber compatible vehicle, said forced impregnation thereby distributing color uniformly among the various chain lengths of said elastomeric material;
• and
• (d) mixing said pigmented elastomeric material with a solvent system to form an erasable ink composition having a viscosity in the range 1.times.10.sup.6 -4.times.10.sup.6 cps, said solvent system comprising a volatile component and an essentially non-volatile component in which the volatile component has a boiling point less than about 180.degree. C. and said essentially non-volatile component is at least predominantly a low viscosity hydrocarbon oil having a viscosity in the range 1-15 cps and a boiling point less than about 300.degree. C. and greater than about 180.degree. C., and which includes less than 15% by weight of the erasable ink composition of a high viscosity material having a boiling point greater than 300.degree. C.

In a ball point writing instrument containing an ink capable of depositing a trace on an absorbent, paper-like writing surface having minute voids therein, said trace being characterized by initial erasability thereof by ordinary pencil erasers and developing permanence over a period of time, said ink comprising pre-pigmented elastomer dispersed in a solvent system, said pre-pigmented elastomer comprising a pigment and an elastomer component in which the elastomer component has a Mooney viscosity within a desired range and the pigment is force impregnated into the elastomer component prior to the pre-pigmented elastomer being dispersed in the solvent system, said pre-pigmented elastomer comprising about 1/3 to 2/3 by weight of the ink and said solvent system comprising a volatile component and an essentially non-volatile component both of which are solvents for the elastomer component and in which the volatile component has a 100% evaporation rate of less than about 60 minutes and said essentially non-volatile component has a 100% evaporation rate of more than about 1200 minutes, said volatile component being present in amount of about 8 to 30% by weight of the ink, said essentially non-volatile component being present in amount of about 21-50.5% by weight of the ink and having a viscosity which is sufficiently low as to result in a final ink viscosity of between 1.times.10.sup.6 -4.times.10.sup.6 cps.