JP2011225852A - Pencil lead - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pencil lead excellent in the balance of bending strength and density.SOLUTION: The pencil lead uses at least a constitutional material and a coupling material as main materials, and is formed by being applied with mixture, molding and burning treatment. The pencil lead contains, in a pencil lead body, inorganic particles other than graphite whose particle diameters are not smaller than 0.1 μm and smaller than 1.0 μm.

Description

  The present invention relates to a pencil lead formed by using an extender and a binder as main materials and performing kneading, molding, and firing.

  Generally, the pencil lead is made of graphite, mica, talc, boron nitride, and other materials, polyvinyl chloride resin, polyvinylidene chloride resin, polyvinyl acetate resin, chlorinated polyethylene resin, polyvinyl alcohol resin, polyacrylamide resin, chlorinated resin. Organic binders such as paraffin resin, phenol resin, furan resin, urea resin, and butyl rubber are used as the main material. If necessary, inorganic additives such as amorphous silica and carbon black, plastics such as phthalate esters, etc. , Stabilizers such as stearates, lubricants such as stearic acid, solvents such as methyl ethyl ketone, these raw materials are dispersed and mixed, kneaded, formed into fine wires, and then subjected to heat treatment as appropriate to the firing temperature. If necessary, silicone oil, liquid paraffin, spindle oil, squalane, α-olefin oil Goma, paraffin wax, microcrystalline wax, polyethylene wax, montan wax, and prepared by impregnating the like oil or waxes such as carnauba wax.

As described above, micron-order inorganic particles such as graphite, mica, talc, and boron nitride are used for the material for the pencil core. Above all, graphite with a developed crystal has a flat (plate-like) shape, When the pencil core is formed into a thin line shape, it is oriented in the extrusion direction of the material to form a skeleton of the core body. This oriented skeleton structure (honeycomb structure) made of graphite improves the strength of the core, and forms the skeleton having the highest strength among those used as the extender. A skeleton formed by a material other than graphite, or a composite skeleton made of graphite and a material other than graphite formed when a material other than graphite and graphite is used, has a lower strength than a skeleton formed solely by graphite. For this reason, the bending strength of the pencil lead is generally reduced.
In addition, the inorganic additive is not necessarily an essential material for the manufacture of the pencil lead, but has the effect of improving the concentration of the pencil lead, improving the dimensional stability, preventing cracking and swelling, and improving the holding force of the mechanical pencil, Nanoparticles such as amorphous silica and carbon black are often preferred and used as inorganic additives. These inorganic additives remain in the core body as the final constituent material of the pencil lead, and have the effect of storing oils to be impregnated thereafter. By using these, the concentration as the pencil lead is improved. be able to. This is because, generally, as the amount of impregnation of the oily material increases, the collapse of the pencil core during writing is promoted and the concentration is improved.
By the way, it is difficult to achieve both the bending strength and the density of the pencil lead. If you try to increase the bending strength, the density will decrease. Conversely, if you try to increase the density, the bending strength will decrease. It is common. Various inventions have been reported to achieve both the bending strength and the concentration of the pencil lead, and there are also related to an extender used as a main material and inorganic additives and lubricants used as necessary.
The invention described in Japanese Examined Patent Publication No. 42-007166 (Patent Document 1) relates to a pencil lead using natural mica as a part of an extender, and disclosed in Japanese Patent Application Laid-Open No. 54-088423 (Patent Document 2). The described invention relates to a pencil lead using talc as a part of the extender, and the invention described in Japanese Patent Application Laid-Open No. 2004-262984 (Patent Document 3) uses plate-like alumina as a part of the extender. The invention described in Japanese Patent Application Laid-Open No. 2008-081715 (Patent Document 4) relates to a pencil core using synthetic mica as a part of an extender, and Japanese Patent No. 4122945. The invention described in (Patent Document 5) relates to a pencil lead using hydrophobic amorphous silica as an inorganic additive, and Japanese Patent Application Laid-Open No. 2007-138031 (Patent Document 6). The invention described relates to a pencil lead using nanomaterials as a lubricant (so-called inorganic additives).

Japanese Patent Publication No.42-007166 Japanese Patent Laid-Open No. 54-088423 JP 2004-262984 A JP 2008-081715 A Japanese Patent No. 4122945 JP 2007-138031 A

The inventions described in Patent Documents 1 and 2 use natural mica and talc as a part of the constitution material, but natural mica and talc have a heat-resistant temperature of about 800 ° C., so heat treatment at a higher temperature is performed. , The crystal expands and decomposes due to the dehydration reaction, resulting in poor cleavage and smoothness, which inhibits core wear when writing with a pencil lead, resulting in extremely low concentration and poor writing. As a result, the high-temperature heat treatment at 1000 ° C. or higher, which is advantageous for improving the strength of the pencil lead, cannot be performed, so that satisfactory bending strength as a pencil lead cannot be obtained.
The inventions described in Patent Documents 3 and 4 use plate-like alumina and synthetic mica having high heat resistance as part of the build material, which is impossible with the inventions described in Patent Documents 1 and 2. This is an invention that enables high-temperature heat treatment as described above.
However, these inventions described in Patent Documents 1 to 4 are not intended to improve the bending strength of the pencil core, but lead color peculiar to graphite by using plate-like particles other than graphite as part of the extender. Since the writing line of the invention is intended to be close to pure black, the amount of plate-like particles other than graphite with respect to the total amount of the extender is at least a high ratio of 10% or more, and occupies the proportion of the extender The ratio of graphite was reduced, and the bending strength as a pencil lead was lowered.
In the inventions described in Patent Documents 5 and 6, ceramic-based or carbon-based nanoparticles (materials) are added and used in an amount of several percent with respect to the total amount of the extender. By using nanoparticles (materials) such as amorphous silica and carbon black described in these, as described above, the pencil core concentration is improved, the dimensional stability is improved, cracking and swelling are prevented, and the mechanical pencil chuck is held. These nanoparticles (materials) are preferably used because of their effects such as improving their strength, but these nanoparticles (materials) have a very large number of particles present in the pencil core even when used in small quantities, and their presence is the result of pencil core molding. It is also true that it sometimes obstructs the orientation of graphite used as an extender in the direction of extrusion, which causes defects in the skeleton formed by graphite in the core, and the bending strength as a pencil core. It was something that would reduce the thickness.
An object of the present invention is to solve these problems and to provide a pencil lead with an excellent balance in which both bending strength and density are compatible.

  The present invention relates to a pencil core obtained by using at least an extender and a binder as main components and subjected to kneading, molding, and firing, and graphite having a particle diameter of 0.1 μm or more and less than 1.0 μm in the pencil core. The gist is a pencil lead containing other inorganic particles.

The pencil core of the present invention contains inorganic particles other than graphite (hereinafter referred to as submicron particles) having a particle size of 0.1 μm or more and less than 1.0 μm in the core, and the submicron particles are When the same weight is used, the number of particles present in the core is remarkably reduced as compared with the aforementioned nanoparticles. For example, nanoparticles (1 to 100 nm) and submicron particles (0.1 to 1 μm = 100 to 1000 nm) have a general density as inorganic particles, and the amount used is about 10% with respect to the total amount of the extender. if less, the number of nanoparticles and sub-micron particles present in the core is ¹⁰ 12 ^{to 10 18, respectively,} 10 9 ^{to 10 12} cells and become, the nanoparticles 1 to 10 ⁹ times the submicron particles Number of particles. Therefore, the number of submicron particles present in the core is 10 ¹² or less, and the possibility of inhibiting the orientation of graphite in the extrusion direction during pencil core molding is less than when nanoparticles are present in the core. Is very small.
In addition, since the submicron particles are not of a size that forms a skeleton with graphite, the skeleton in the core is almost a skeleton of graphite alone even if it is located in a gap formed by contact between the extender materials. The core strength by a single skeleton can be obtained. As a result, the orientation of graphite in the extrusion direction can be obtained, and a strong skeleton core can be formed with the strength of the skeleton caused by the graphite.
In addition, the submicron particles are arranged so as to fill a gap formed by contact between the constituent materials, and function to store an oily substance to be impregnated later. The fine gaps formed in the gaps between the build material and the submicron particles exert capillary force and contribute to the retention of the impregnated oil, and the submicron particles themselves retain the oil. If it is porous, it can greatly contribute to the retention of oil.
Thus, in order to fill the gap formed by the contact between the extender materials, it is effective that the particle diameter of the inorganic additive is smaller than the size of the gap, but the extender having a diameter of about 10 μm to 30 μm. Since the submicron particles have a particle diameter of 0.1 μm or more and less than 1.0 μm with respect to the material, the particle diameter is almost 1/10 or less of the particle diameter of the extender. It can be located in the gap formed by contact, and the function when submicron particles are used as a functional additive can be sufficiently exhibited.
By the way, in general, when two particles having different sizes (assuming a sphere) have a large / small particle radius ratio larger than 1: 0.155 (= 6.45: 1), a plurality of particles Since it is theoretically possible for small particles to be located in the smallest space formed by the contact of large particles, the particle size of the inorganic additive is also the constitution of inorganic additives and extenders that are not spheres. If it is 1 / 6.45 or less of the particle diameter of a material, it is thought that possibility that an inorganic additive will be located in the clearance gap formed by the contact between extenders becomes high.
In addition, if the submicron particles have a plate shape similar to that of graphite, the particles exhibit the same behavior as graphite when forming a pencil core, and the possibility of inhibiting the orientation of graphite in the extrusion direction is minimized. Therefore, the plate shape is preferable.
In addition, since the heat-resistant temperature of the submicron particles is 1000 ° C. or higher, even if a high temperature heat treatment at 1000 ° C. or higher, which is advantageous for improving the strength of the pencil lead, is applied, Changes in characteristics such as smoothness are small, and the density of the pencil lead is low and the writing quality is unlikely to deteriorate.

Details will be described below.
As described above, the submicron particles contained in the pencil core of the present invention do not hinder the orientation of graphite in the extrusion direction at the time of pencil core molding, and at the same time, the effect as an inorganic additive is also exhibited. It is premised on the submicron order in the core.
Various inorganic particles other than graphite can be cited as the inorganic particles that form the submicron particles. For example, natural mica, synthetic mica, talc, boron nitride, Examples include plate-like alumina, scaly low crystalline silica, and amorphous plate-like silica.
The submicron particles may be present in any shape, but if the plate shape is similar to that of graphite, the particles exhibit the same behavior as graphite during pencil core molding, and the graphite is oriented in the extrusion direction. Since the possibility of hindering this is reduced as much as possible, a plate shape is preferred. In general, if the aspect ratio (maximum particle diameter / minimum particle diameter) is 5 or more, it is said to be a plate shape, but if the aspect ratio becomes too large, the bending strength of the particles themselves decreases, so the aspect ratio is About 5-20 is desirable.

In addition, as for the heat resistance of the submicron particles, some of them can be used if they are heat-treated at temperatures lower than 800 ° C., such as natural mica and talc. It is only necessary to selectively use the inorganic particles that are the basis of the particles. However, as described above, if the heat resistance temperature of the submicron particles is 1000 ° C. or higher, high-temperature heat treatment advantageous for improving the strength of the pencil core can be performed. The change in properties such as the smoothness of the particle that affects the pencil core concentration and writing quality is generally small, and the pencil core concentration is low and the writing quality is less likely to deteriorate. It is preferable to selectively use synthetic mica, plate-like alumina, scaly low crystalline silica, amorphous plate-like silica or the like having a temperature of 1000 ° C. or higher.
However, even if the heat-resistant temperature is the same, changes in properties such as the pencil core concentration and particle smoothness that affect the writing quality vary depending on the substance, so the angle of repose, which is one of the measures of powder particle smoothness, is changed. It is better to compare before and after heat treatment. In general, graphite particles used as an extender have very high heat resistance and smoothness in an inert atmosphere, which expresses good writing quality as a pencil lead, but the angle of repose of graphite is generally In a heat treatment at 800 ° C. to 1300 ° C. in an inert atmosphere, which is a typical manufacturing condition for a pencil lead, a value that does not change at all from that before the heat treatment (55 ° before heat treatment, 55 ° after heat treatment at 1100 ° C.) Then, the submicron particles in the core remain in the vicinity of the graphite in the core as the final component of the pencil core, and are written and worn on the paper together with the graphite when writing the pencil core. In order to influence, the value of the angle of repose after heat treatment is preferably as small as possible, and particularly preferably less than 60 °. Further, the angle of repose before the heat treatment does not affect the writing quality of the pencil lead, but if it is too large, the flow of the material at the time of molding may be hindered. Those less than ° are particularly preferred. That is, it is preferable that the angle of repose before heat treatment is small and the change in the angle of repose is small regardless of the heat treatment temperature.
Regarding the change in the angle of repose of the various inorganic particles described above in the heat treatment at 800 ° C. to 1300 ° C. in an inert atmosphere, which is a general manufacturing condition for a pencil lead, there is almost no change in amorphous plate-like silica (heat treatment). In contrast to the previous 53 °, 52 ° after heat treatment at 1100 ° C., other inorganic particles often change their repose angle to a large value as the crystallization progresses at 1000 ° C. or higher. For example, 55 ° before heat treatment is 60 ° after heat treatment at 1100 ° C. for talc, and 63 ° before heat treatment is 65 ° after heat treatment at 1100 ° C. for synthetic mica. This difference is considered to be due to the difference between crystalline particles and amorphous particles, but not only is the repose angle measured with inorganic particles alone, but is also close to the actual blending ratio of pencil lead, for example, graphite. The difference clearly occurs even in a mixed powder of 98% and inorganic particles 2%.
As for the angle of repose, since the value after heat treatment affects the density and writing quality of the pencil lead, it is preferably as small as possible and particularly preferably less than 60 °. Further, the value before the heat treatment does not affect the concentration and writing quality of the pencil lead, but if it is too large, the flow of the material at the time of forming the pencil lead may be hindered. Therefore, from the viewpoint of heat resistance, it is most preferable to select and use amorphous plate-like silica among inorganic particles.
The angle of repose was measured according to JIS R9301-2-2.

Since it is difficult to confirm the particle size of the inorganic particles contained in the pencil core as it is, inorganic particles obtained by heat-treating the pencil core after firing in an oxidizing atmosphere and carbonaceous materials such as graphite are not oxidized and consumed. It is better to confirm by observing the ash content of the particles. The obtained ash content is ash content of the inorganic particles alone after the skeleton formed by graphite has disappeared due to oxidative exhaustion, and often has a shape similar to a pencil core. At that time, since the ash state may change due to the difference in heat treatment conditions, it is desirable to adopt a certain heat treatment condition for the evaluation. The condition adopted in the present invention is that the pencil core after the baking treatment to be observed is placed on a magnetic dish and heat-treated at a heating rate of 10 ° C. per minute from room temperature to 800 ° C. in an oxidizing atmosphere, and the maximum temperature of 800 ° C. And then cooling for 1 hour.
A method for confirming that the inorganic particles are submicron particles is to measure the particle diameter of the existing inorganic particles on a photograph of ash taken with an electron microscope. Of the major axis, minor axis, and thickness of the particles, the maximum value is the maximum particle size, and the minimum value is the minimum particle size. Hereinafter, the particle diameter of the inorganic particles refers to the maximum particle diameter. Photographed at three locations on the side and cross-section of the ash with an electron microscope, arbitrarily selected 50 inorganic particles on the photograph, measured the particle size, nanoparticles (less than 0.1 μm), The quantity ratio is calculated by classifying into submicron particles (0.1 μm or more and less than 1.0 μm) and micron particles (1.0 μm or more). Of these, the aspect ratio (maximum average particle diameter / minimum average particle diameter) is calculated from the maximum average particle diameter and the minimum average particle diameter of the submicron particles. An imaging magnification of about 10,000 times is sufficient for confirmation of submicron particles, but about 50,000 times are necessary for confirmation of nanoparticles.
As the content ratio of inorganic particles by particle size with respect to the ash content of the pencil core, the higher the total content ratio of nanoparticles and micron particles, the lower the bending strength of the pencil core. The content ratio of is preferably 40% by weight or more. The content ratio of each particle is calculated from the number ratio of the inorganic particles by particle diameter determined from the electron micrograph, the blending amount of the used inorganic particles, and the weight of ash. For example, when only graphite is used as an extender and only amorphous silica of nanoparticles is used as an inorganic additive, the resulting ash content is only amorphous silica. % Content ratio. Also, if graphite and micron particle talc are equally used as extender, and if talc is all micron particles without using inorganic additives, the ash content is only talc. In contrast, the content ratio of micron particles is 100% by weight.

The submicron particles contained in the pencil core may be inorganic particles having an average particle diameter of the order of microns at the time of blending. However, in order to express the effect as an inorganic additive, the particles of inorganic particles are used in the molding process. Since there is little possibility that the diameter will change, it is necessary to prepare submicron particles in the dispersion mixing process and kneading process up to the molding process. In order to make micron-order inorganic particles into submicron particles, it is possible to increase the load strength in the dispersion mixing process and kneading process. However, the generation ratio of nanoparticles is larger than that of submicron particles at excessively high load strength. Therefore, the conditions should be set in consideration of the impact resistance of the inorganic particles used. For example, plate-like alumina is a plate-like particle having pseudo-cleavage in which flaky particles simply adhere to each other, and has a high heat resistance temperature but is not so strong as other inorganic particles. Dispersion mixing and kneading are not preferred. Incidentally, in the dispersion mixing process using a Henschel mixer or the like, the inorganic particles to be blended are likely to be so-called volume pulverization, which is similar to the original shape. For this reason, the inorganic particles to be blended are thinner than the original shape and have a shape with a high aspect ratio, so that the so-called area pulverization is easy, and kneading with a triple roll having a strong shearing force is particularly preferable for increasing the aspect ratio. .
If kneading with this roll is carried out for a long time, the area pulverization of inorganic particles will progress and the number of submicron particles will increase, but the pulverization of other materials such as build materials and binders will progress too much and the overall particle size balance will be lost, which is inconvenient When this occurs, excessive pulverization of other materials such as the extender and the binder can be suppressed by reducing the load strength of the roll for a long time. Alternatively, only the inorganic particles to be used may be roll-treated in advance, and then mixed with the remaining materials and roll-treated again. By doing so, materials other than inorganic particles are not crushed more than necessary.
As for the heat treatment conditions for the pencil lead, the molding as a pencil lead has already been completed, so it does not affect that much, but the higher the maximum temperature, the greater the angle of repose due to the sintering of inorganic particles, so the highest temperature Is preferably kept below the sintering temperature of the inorganic particles. It is preferable to grasp the conditions suitable for the purpose while confirming the particle diameter and content ratio of the inorganic particles contained in the pencil core obtained according to the conditions in each step.
Also, as the amount of inorganic particles used increases, the absolute amount of nanoparticles and micron particles also increases, and the bending strength of the pencil core decreases even if the content of submicron particles in the ash is 40% by weight or more. Since the possibility increases, it is preferable that the amount of inorganic particles used is approximately 5% or less with respect to the total amount of the extender. In addition, nano particles such as amorphous silica and carbon black are used in combination with inorganic additives to increase the effect of improving the pencil core concentration, improving dimensional stability, preventing cracking and swelling, and improving the chuck holding power of mechanical pencils. However, in that case, since the bending strength of the pencil lead decreases as the amount used increases, it is desirable to suppress the amount used to less than 1% with respect to the total amount of the build material.

Conventionally known materials can be used as materials other than those described above. Examples of the extender include one or more selected from general scale-like graphite, scale-like graphite, earth-like graphite, artificial graphite, mica, talc, boron nitride, and the like. As binders, organic binders such as polyvinyl chloride resin, polyvinylidene chloride resin, polyvinyl acetate resin, chlorinated polyethylene resin, polyvinyl alcohol resin, polyacrylamide resin, chlorinated paraffin resin, furan resin, urea resin, butyl rubber, etc. 1 type or 2 types or more selected from among them can be exemplified.
Furthermore, if necessary, inorganic additives such as amorphous silica and carbon black, dioctyl phthalate, dibutyl phthalate, tricresyl phosphate, dipropylene glycol dibenzoate, dioctyl adipate, propionate carbonate, stearic acid, etc. A stabilizer such as salt, a lubricant such as stearic acid, a solvent such as methyl ethyl ketone, and the like can be used in combination as appropriate.

  These raw materials are dispersed and mixed with a Henschel mixer, etc., kneaded with a kneader, three rolls, etc., then extruded into a thin line, heat treated from room temperature to around 300 ° C. in air, and then in an inert atmosphere. A baking treatment at 800 ° C. to 1300 ° C. is performed, and if necessary, silicone oil, liquid paraffin, spindle oil, squalane, α-olefin oligomer, paraffin wax, microcrystalline wax, polyethylene wax, montan wax, carnauba wax, etc. A pencil lead is produced by appropriately impregnating oil or wax. In addition, if necessary, pigments, dyes, and the like may be used in combination to form a colored pencil lead.

EXAMPLES Hereinafter, although this invention is demonstrated based on an Example, this invention is not limited to an Example.
<Example 1>
Graphite (extrinsic material, average particle diameter: 20 μm) 73 parts by weight polyvinyl chloride resin (binder) 55 parts by weight Silleaf (amorphous plate silica of Mizusawa Chemical Co., Ltd., average particle diameter: 6 μm, heat-resistant temperature: 1100 ° C) 2 parts by weight dioctyl phthalate (plasticizer) 25 parts by weight stearate (stabilizer) 2 parts by weight stearic acid (lubricant) 1 part by weight methyl ethyl ketone (solvent) 20 parts by weight Treatment, kneading treatment with three rolls, extrusion molding into a thin wire, heating from room temperature to 300 ° C. in air over about 10 hours, heat treatment for holding at 300 ° C. for about 1 hour, Then, a baking treatment at a maximum of 1100 ° C. was performed in a closed container, and after cooling, liquid paraffin was impregnated to obtain a pencil lead having a nominal diameter of 0.5.

<Example 2>
In Example 1, instead of Sylleaf, Seraph YFA05025 (YKK Co., Ltd. plate-like alumina, average particle size: 5 μm, heat resistance temperature: 1200 ° C.) was used in the same manner as in Example 1 except that 2 parts by weight were used. Thus, a pencil lead having a nominal diameter of 0.5 was obtained.

<Example 3>
In Example 1, PDM-9WA (fluorine phlogopite manufactured by Topy Industries Co., Ltd., average particle size: 12 μm, heat resistant temperature: 1100 ° C.) was used in place of Silleaf, except that 2 parts by weight were used. Similarly, a pencil lead having a nominal diameter of 0.5 was obtained.

<Example 4>
In Example 1, 2 parts by weight of CT35 (Talc manufactured by Yamaguchi Mica Kogyo Co., Ltd., average particle size: 15 μm, heat-resistant temperature: 800 ° C.) was used instead of silleaf, and the maximum temperature of the baking treatment was 800 ° C. A pencil lead having a nominal diameter of 0.5 was obtained in the same manner as in Example 1 except for the above.

<Examples 5-7>
In Example 1, the nominal diameter was changed in the same manner as in Example 1 except that the kneading treatment time was changed to 1.5 times, 1.2 times, and 0.8 times respectively without changing the load strength of the three rolls. A pencil lead of 0.5 was obtained.

<Example 8>
In Example 1, a pencil core having a nominal diameter of 0.5 was obtained in the same manner as in Example 1 except that the load strength of the three rolls was increased 0.8 times and the kneading time was increased 1.5 times. .

<Example 9>
In Example 1, the same as in Example 1, except that the dispersion mixing processing time by the Henschel mixer was 1.5 times and the kneading processing time was 1.5 times without changing the load strength of the three rolls. A pencil lead with a diameter of 0.5 was obtained.

<Examples 10 to 13>
In Example 1, the amount of sylleaf used was changed to 4 parts by weight, 3.5 parts by weight, 1 part by weight and 0.7 parts by weight, respectively, and the amounts of graphite used were 71 parts by weight, 71.5 parts by weight and 74 parts by weight, respectively. A pencil lead having a nominal diameter of 0.5 was obtained in the same manner as in Example 1 except that the amount was changed to parts by weight and 74.3 parts by weight.

<Example 14>
In Example 1, a pencil lead having a nominal diameter of 0.5 was obtained in the same manner as in Example 1 except that the baking process in the sealed container was changed to the baking process that maximizes 1200 ° C.

<Comparative Example 1>
Example 1 except that 2 parts by weight of Aerosil R972 (hydrophobic amorphous silica manufactured by Nippon Aerosil Co., Ltd., average particle size: 16 nm, heat resistant temperature: 1600 ° C.) was used in place of Silleaf in Example 1. In the same manner, a pencil lead having a nominal diameter of 0.5 was obtained.

<Comparative example 2>
In Example 1, a pencil lead having a nominal diameter of 0.5 was obtained in the same manner as in Example 1 except that the amount of graphite used was changed to 75 parts by weight without using sill leaf.

As described above, the bending strength and the concentration were measured according to JIS S 6005 for the pencil lead obtained in each of the examples and comparative examples. In addition, each pencil lead is placed on a magnetic dish, heat-treated in an oxidizing atmosphere from room temperature to 800 ° C. at a heating rate of 10 ° C. per minute, held at the maximum temperature of 800 ° C. for 1 hour, and then cooled down to ash content. Got. For each of the obtained ash side and cross-section, photographed at 10000 times with an electron microscope at three different locations, arbitrarily selected 50 inorganic particles on the photograph, and measured the particle size. The quantity ratio by particle size was calculated by classifying into nanoparticles (less than 0.1 μm), sub-micron particles (more than 0.1 μm, less than 1.0 μm), and micron particles (more than 1.0 μm). Of these, the aspect ratio (maximum average particle diameter / minimum average particle diameter) was calculated from the maximum average particle diameter and the minimum average particle diameter of the submicron particles.
Furthermore, the content of submicron particles relative to the ash content was calculated from the ratio by number of inorganic particles determined from the photograph, the blending amount of the inorganic particles used, and the weight of the ash. The results are shown in Table 1.

Table 1 will be described.
Example 1 is an example in which the most preferable amorphous plate-like silica is used as the inorganic particles, and has excellent values in which bending strength and concentration are very balanced.
Example 2 is an example in which plate-like alumina is used as inorganic particles, and the impact resistance of plate-like alumina is lower than that of amorphous plate-like silica, resulting in a decrease in bending strength as a pencil core. It has become.
Example 3 is an example in which fluorine phlogopite is used as inorganic particles, and the repose angle of fluorine phlogopite is larger than that of amorphous plate-like silica, resulting in a decrease in the density as a pencil lead. Yes.
Example 4 is an example in which talc is used as the inorganic particles. The heat resistance temperature of talc is low, and the firing temperature is lowered to 800 ° C. accordingly, resulting in a decrease in bending strength as a pencil lead. Yes.
Examples 5 to 7 are examples in which the kneading treatment time was changed to 1.5 times, 1.2 times, and 0.8 times of Example 1 without changing the load strength of the roll, respectively. The longer the aspect ratio of the inorganic particles, the higher the bending strength as a pencil core, and the shorter the kneading time, the lower the aspect ratio of the inorganic particles, so the bending strength is reduced.
Example 8 is an example in which the load strength of the roll is 0.8 times that of Example 1 and the kneading treatment time is 1.5 times longer. The bending strength of the pencil lead is more than the kneading treatment time of the roll. Since the influence of the load strength is large, the bending strength decreases and the concentration increases.
Example 9 is an example in which the dispersion mixing treatment time by the Henschel mixer is 1.5 times that of Example 1, and the kneading treatment time is 1.5 times without changing the load strength of the roll. By increasing the mixing treatment time, the average particle size of the inorganic particles is reduced, and both the bending strength and the concentration as a pencil core are reduced.
Examples 10 to 13 are examples in which the amount of sylleaf used as inorganic particles was changed to 4, 3.5, 1.0, and 0.7 parts by weight. Since the amount of particles increases and the possibility of inhibiting the orientation of graphite at the time of pencil core molding increases, the bending strength as the pencil core tends to decrease and the concentration tends to increase. In Example 10, since the amount of inorganic particles used exceeds 5% of the total amount of the extender, the bending strength is greatly reduced and the concentration is not increased.
Example 14 is an example in which the firing temperature is 1200 ° C., and since the heat resistance temperature of the silleaf is exceeded, the smoothness of the particles is lowered by sintering the silleaf, and the bending strength as a pencil core is increased. The result is a decrease in concentration.
Comparative Example 1 is an example in which Aerosil R972 is used as inorganic particles instead of sylleaf, and the orientation of graphite at the time of pencil core molding is hindered, resulting in a decrease in bending strength as a pencil core.
Comparative Example 2 is an example in which the amount of graphite used is increased without using inorganic particles other than graphite, and since the orientation of the graphite at the time of pencil core molding is not hindered, the bending strength as a pencil core is high, but an oily substance is used. Since there are no inorganic particles that have the effect of accumulating, the concentration is greatly reduced.
As described above, in a pencil core formed by using an extender and a binder as main materials and subjected to kneading, molding, and firing, graphite having a particle diameter of 0.1 μm or more and less than 1.0 μm in the pencil core. By containing inorganic particles other than the above, it is possible to provide a pencil lead having an excellent balance between bending strength and concentration.

Claims (4)

In a pencil core formed by using at least an extender and a binder as main materials and subjected to kneading, molding and firing, inorganic particles other than graphite having a particle diameter of 0.1 μm or more and less than 1.0 μm in the pencil core Containing pencil lead. The pencil lead according to claim 1, wherein the content of the inorganic particles is 40% by weight or more based on the ash content of the pencil lead. The pencil lead according to claim 1 or 2, wherein the inorganic particles have a plate shape.   The pencil lead according to any one of claims 1 to 3, wherein the inorganic particles are inorganic particles having a heat resistant temperature of 1000 ° C or higher.