Baked color pencil lead and process for producing the same

Compositions: coating or plastic – Coating or plastic compositions – Marking

Reexamination Certificate

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C106S287120, C106S287130, C106S287140

Reexamination Certificate

active

06709501

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a baked color pencil lead which is excellent in mechanical strength (bending strength, tensile strength, impact strength and the like) while exhibiting a satisfactory coloring power and sufficient drawn line density and a process for producing the same, particularly to a baked color pencil lead which is suited to a color pencil lead for a mechanical pencil and a wood case pencil and to a process for producing the same.
2. Description of the Related Art
In a conventional baked color pencil lead, clay and the like are used as a binder, and a filler such as boron nitride and, if necessary, a heat resistant pigment and a reaction-accelerating agent are added thereto to prepare a blend composition. The blend composition is kneaded and extruded, and then it is subjected to heat treatment to obtain a porous baked lead. Pores of this lead are filled with an ink comprising a dye and a pigment to prepare a color pencil lead, which has been a leading color pencil lead.
Important characteristics usually required for a color pencil lead are high mechanical strength, a good coloring power of drawn lines and strong density of drawn lines, but in the baked color pencil lead described above, mechanical strength is not sufficiently high, and strong drawn line density and a good coloring power are not obtained. Accordingly, desired is a color pencil lead in which the lead has large porosity so that a sufficiently large amount of a colorant can be contained and which has sufficiently high mechanical strength.
Known as a method for solving these problems is a baked color pencil lead prepared by impregnating pores of a baked lead comprising at least a filler with a perhydropolysilazane solution to form silicon nitride as a binder by heat treatment in an inert atmosphere and filling pores of the lead with an ink, and a process for producing the same (Japanese Patent Application Laid-Open No. 48931/1996 filed by the present applicants).
Further, a process for producing a baked color pencil lead comprising the steps of baking a lead at 1000° C. or higher to form silicon nitride in the lead and then subjecting it to heat treatment at 600° C. or higher in an oxidation atmosphere is known as a method of solving a problem on darkening of a baked lead and a low chroma thereof which are caused by carbonization of a trace amount of an organic solvent remained in the lead (Japanese Patent Application Laid-Open No. 119582/2000 filed by the present applicants).
Baked color pencil leads obtained by the methods disclosed in these respective prior applications are excellent in mechanical strength while exhibiting a satisfactory coloring power and sufficient drawn line density which have so far never been seen, but there are a few problems in that once silicon nitride is formed, a compound in which a carbon atom chemically combined with a silicon atom and residual carbon covered with silicon nitride can not completely be removed even by subsequent baking in an oxidation atmosphere, which causes darkening in lines drawn by a baked color pencil lead for a light color.
Further, provided as a baked color pencil lead having high chroma by the present applicants are a baked color pencil lead prepared by impregnating pores of a baked lead comprising at least a filler with a polysilazanes-containing solution, subjecting it to heat treatment in the air to thereby form silicon oxide as a binder and filling pores of the lead with an ink, and a process for producing the same (Japanese Patent Application Laid-Open No. 17221/2000).
However, when silicon oxide is formed as a binder by the method described above, pores formed after baking have a small diameter and a small pore volume, and therefore there is a problem in that writing performance is a little hard and such a lead is not equal to a silicon nitride lead in coloring power.
On the other hand, known are a baked color pencil lead using as a binder, a compound obtained by heating at least one selected from the group consisting of organozirconium compounds and organoaluminum compounds (Japanese Patent Application Laid-Open No. 67540/1997) and a baked color pencil lead prepared by impregnating pores of a baked lead comprising at least a filler with a solution of at least one selected from the group consisting of metal chelates, metal carboxylates and metal alkoxides, subjecting the lead to heat treatment to thereby form metal oxide as a binder and filling pores of the lead with an ink, and a process for producing the same (Japanese Patent Application Laid-Open No. 343445/1999).
However, when using the organic compounds and the metal chelates disclosed in the respective prior applications described above, there are the problems in that it is difficult to control reaction in kneading, extruding and impregnating into pores and that unevenness is caused to a solution concentration and a viscosity in the course of impregnation, so that strength and hardness of the baked lead are liable to be dispersed.
In light of the conventional problems described above, an object of the present invention is to further improve the above-mentioned prior arts disclosed by the present inventors and to provide a baked color pencil lead having markedly excellent mechanical strength, particularly a vivid and excellent coloring power and smooth writing performance while using silicon oxide as a binder, and a process for producing the same.
SUMMARY OF THE INVENTION
Intensive researches repeated by the present inventors in order to solve the conventional problems described above have resulted in finding that the problems described above can be solved by impregnating pores of a porous baked lead comprising at least a filler with a solution containing a polysiloxane having a specific structural unit and subjecting it to heat treatment in an oxidation atmosphere and/or a non-oxidation atmosphere, and thus the present invention has come to be completed.
That is, the baked color pencil lead of the present invention and a process for producing the same comprise the following items (1) to (3).
(1) A baked color pencil lead comprising a baked lead obtained by impregnating pores of a porous baked lead comprising at least a filler with a solution containing a polysiloxane having a structural unit represented by the following Formula (I) and subjecting it to heat treatment and a colorant filled into pores of the resultant baked lead:
wherein R
1
and R
2
each represent an alkyl group, an alkoxy group, a hydroxy group or a siloxane group, and they may be the same or different.
(2) The baked color pencil lead as described in the above item (1), wherein the porous baked lead comprising a filler described above is a porous baked lead obtained by kneading and extruding a blend composition comprising at least a filler and an organic excipient, baking it in a non-oxidation atmosphere to obtain a first baked lead and further heating the first baked lead in an oxidation atmosphere to remove a carbon content by oxidation.
(3) A process for producing a baked color pencil lead comprising the steps of:
kneading and extruding a blend composition comprising at least a filler and an organic excipient and then baking it in a non-oxidation atmosphere to obtain a first baked lead,
further heating the above first baked lead in an oxidation atmosphere to remove a carbon content by oxidation to obtain a second baked lead,
impregnating pores of the above second baked lead with a solution containing a polysiloxane having a structural unit represented by the following Formula (I) and subjecting it to heat treatment in an oxidation atmosphere and/or a non-oxidation atmosphere to thereby obtain a third baked lead, and
filling pores of the above third baked lead with a colorant:
wherein R
1
and R
2
each represent an alkyl group, an alkoxy group, a hydroxy group or a siloxane group, and they may be the same or different.
According to the present invention, provided are a baked color pencil lead in which silicon oxide is uniformly present in the whole p

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