JPH07258594A - Pencil lead and its production - Google Patents

Abstract

PURPOSE:To obtain a pencil lead which has such high bending strength as not to be attained conventionally without using graphite, can give a drawn line having low glossiness and a bluish black color, and has smoothness in writing. CONSTITUTION:The fired pencil lead containing boron nitride as a substantial portion of the crystalline lubricant for an inorganic extender, wherein the nitride substantially consists of boron nitride having a graphitization index [{(100)+(101)}/(102)] represented in terms of a ratio of the areas of peaks in X-ray diffractometry of 10 or below and a mean particle diameter of below 8.0mum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pencil lead, particularly a lead for mechanical pencils, and more particularly to a pencil lead having a pure black writing line, a low glossiness and an excellent bending strength, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a pencil lead is prepared by mixing an inorganic substance such as graphite, boron nitride, talc, mica, and carbon black with clay and water as a surfactant or a plasticizer, kneading them, and molding them at a high temperature. To obtain a sintered body, which is then impregnated with fats and oils to produce a clay type product, and the inorganic body material and pitches such as synthetic resin, natural resin or asphalt, a plasticizer and a lubricant. A carbon type obtained by mixing, kneading, molding, and then firing in a non-oxidizing atmosphere at 800 to 1,400 ° C. to carbonize the resin to obtain a fired body using carbon as a binder, and impregnating it with oils and fats. It is roughly divided into ones. Basically, the clay-type pencil lead is used for wood-axis pencils, and the carbon-type pencil lead is used for mechanical pencils because of its practical strength and cost.

Conventionally, a pencil core is made of graphite, which acts as a crystalline lubricant as an extender and coloring agent, and the writing feel and bending strength are improved by the process design such as its composition and orientation. I came. However, since graphite is used as the main coloring material, the broken abrasion powder is rubbed horizontally on the paper surface during writing, and the basal surface of the graphite crystals is aligned horizontally on the paper surface. For this reason, there is a problem that the drawn line has light reflection on the basal plane of graphite, which makes it difficult to read depending on the angle at which the drawn line is seen and the drawn line color is black gray.

Various attempts have been made to bring the drawn line color closer to pure black, but the method of adding carbon black has been mainly studied. Carbon black
It is well known that it is used as an ink pigment for ballpoint pens and felt-tip pens, and has little blackness and reflection of light.
Carbon black, like graphite, is composed almost entirely of carbon, and is basically a substance having a structure in which fine graphite crystallites having a crystallite size of several tens of angstroms are arranged in a spherical shape, and is inferior in lubricity. It has a relatively large number of functional groups on the surface and is surface active.

On the other hand, the graphite used for the pencil lead is natural flake graphite, natural flake graphite, quiche graphite, etc., and is a substance in which graphite crystallites having a crystallite size of 1,000 angstroms or more are laminated and arranged. It has lubricity. The surface has very few functional groups and is inactive. In a pencil lead using graphite, when the entire amount of graphite is replaced with carbon black, the amount of binder resin becomes insufficient with respect to the specific surface area of carbon black, resulting in a pencil lead with extremely low bending strength.

On the contrary, when a pencil lead is made with an appropriate amount of binder resin with respect to the specific surface area of carbon black, carbon black is surface-active, so that when the binder resin is carbonized, it is firmly bonded and the lubricity is improved. Because there is nothing, it becomes a pencil lead that cannot be written at all. When graphite and carbon black are used together, a large amount of carbon black must be used in order to develop the hue of carbon black. For the same reason as when carbon black is used alone, the bending strength It will be low.

When the amount of carbon black used is small, not only does the color of carbon black not appear, but a spherical carbon black is sandwiched between plate-like graphite particles, resulting in a coarse structure. Bending strength is not expressed. Further, although the bending strength is low, the carbon black is firmly bonded to the carbonized product of the binder resin, so that the concentration becomes low. As described above, it is difficult to make the writing line of the pencil lead a pure black hue by using carbon black.

It has long been known that when boron nitride, talc, or mica is used in place of graphite, the color of the carbonized product of the binder resin develops, and it shows a pure black color. However, talc and mica were burned at a high temperature. At this time, the water of crystallization is released to lose the lubricity of the powder, resulting in poor writing quality, which is not practical. Although a method of using boron nitride having a structure similar to that of graphite and excellent in lubricity and heat resistance is known, JP-B-5-9472 and JP-B-5-1039.
No. 7, the bending strength is 196 to 231 MPa, and there is a problem that the bending strength is still insufficient as compared with the core using graphite.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a pencil lead having a pure black writing line, a low glossiness, an excellent writing quality and an excellent bending strength, and a method for producing the same.

[0010]

As a result of intensive studies to solve the above problems, the present inventors have found that the crystallinity is high and the particle size is relatively large in view of the difference between graphite and boron nitride. By using a small boron nitride fine powder, a pencil lead with pure black writing lines, low gloss, and excellent writing characteristics, but with bending strength comparable to a firing core using graphite was obtained. The present invention has been completed.

That is, according to the present invention, (1) in a fired pencil lead using substantially boron nitride as a crystalline lubricant of an inorganic material, boron nitride is substantially expressed by a ratio of X-ray diffraction peak areas. Graphitization index [{(100) + (1
01)} / (102)] is 10 or less and the average particle diameter is less than 8.0 μm. (2) The content of the inorganic substance that is substantially boron nitride is
When the balance is graphite, boron nitride is 80% by weight or more,
When the balance is talc and / or mica, the pencil lead according to the above (1) is 50% by weight or more of boron nitride. (3) Boron nitride reacted at a temperature of 1,500 ° C. or higher or boron nitride reacted at a temperature of less than 1,500 ° C. is again heat-treated at 1,500 ° C. or higher and has an average particle diameter. Is more than 8.0 μm, the obtained powder is pulverized to an average particle size of less than 8.0 μm before use.

Graphite has been used for a long time as a body material and a coloring material for a pencil lead because of its excellent lubricity, heat resistance and anisotropy. There are natural graphite and artificial graphite in graphite,
Natural graphite is classified into scaly graphite, scaly graphite, and earthy graphite. Among them, the ones mainly used for pencil lead are natural flake graphite, natural flake graphite, and quiche graphite classified into artificial graphite, and graphites having high crystallinity are all used.

Since these highly crystalline graphites are produced as large particles of several tens of μm or more, they are pulverized before use to be adjusted to a desired particle size. Therefore, the crystallinity is a high crystal having a graphitization index of 1.0 to 1.3 even after crushing. Here, the graphitization index is a ratio of peak areas of X-ray diffraction [{(100) + (101)} / (102)].
In the parameter represented by, the closer the value is to 1.0, the higher the crystallinity, and the larger the value, the lower the crystallinity.

On the other hand, boron nitride is a raw material produced by reacting a boron source such as boric acid, borax and boron oxide and a nitrogen source such as nitrogen, ammonia and melamine at a temperature of 800 to 2,000 ° C. However, the graphitization index usually strongly depends on the reaction temperature. Generally, in order to set the graphitization index to 10 or less, it is necessary to react at a temperature of 1,500 ° C. or higher, and the boron nitride synthesized in this way has a large average particle size. Boron nitride reacted at less than 1,500 ° C. has an undeveloped crystal because it is used for producing a boron nitride sintered body. The boron nitride sintered body is processed at a high temperature around 2,000 ° C. in the production thereof. At that time, the crystal growth of boron nitride advances the sintering and exhibits excellent performance. The commercially available grades of boron nitride having a small particle size are used for producing a boron nitride sintered body.
Therefore, the graphitization index exceeds 10 and the crystallinity is low.

In the production of the pencil lead, since the firing temperature is around 1,000 ° C. as described above, even if a boron nitride powder having a low crystallinity is used, the crystal growth of the boron nitride particles does not occur and the low crystallinity nitriding occurs. Boron particles will remain in the core as they are and sufficient bending strength will not be obtained, but in some cases, impurities such as boric acid remaining in boron nitride and the binder resin carbonized product will be strongly bonded, resulting in bending. Despite its low strength, it is hard and becomes a core that gives a crisp brushstroke. In addition, commercially available high-crystallinity grade boron nitride powder has a large particle size and a bending strength of JP-B-5-947.
Although it is about the same level as 196 to 231 MPa reported in Japanese Patent Publication No. 2 and Japanese Patent Publication No. 5-10397, it is still insufficient.

Therefore, when the present inventors use a boron nitride powder of a grain size of undeveloped particles having a small particle size for a pencil lead, it is heated again at 1,500 ° C. or more to develop crystals. I thought about it. In the case of growing a crystal by reheating and in the case of synthesizing boron nitride at 1,500 ° C. or higher, grain growth occurs with the growth of the crystal. The degree of grain growth varies depending on the activity of the grain surface and the content of impurities, but as a result, the average grain size may exceed 8.0 μm in some cases. In such a case, the obtained powder is pulverized to have an average particle size of less than 8.0 μm to obtain boron nitride having a graphitization index of 10 or less and an average particle size of less than 8.0 μm. The present invention has been completed by finding that the improvement of bending strength can be achieved by using the obtained boron nitride fine powder having high crystallinity. It is not clear why the pencil lead using boron nitride has a lower glossiness of the drawn line than the pencil lead using graphite,
In the case of graphite, there are pi-electrons due to sp ² hybrid orbitals, and since these pi-electrons have properties close to free electrons, a metallic luster is developed and light reflection on the particle surface occurs. It is considered that in the case of boron nitride, since it does not have pi electrons, light reflection on the particle surface is reduced.

The effect similar to that obtained by using only the boron nitride of the present invention is obtained when the balance of the inorganic material is graphite and the boron nitride content is 80% by weight or more. Two
Although the addition of graphite in an amount of more than 0% by weight slightly improves the strength, the drawn line exhibits a graphite color and the glossiness also increases. Therefore, among the objects of the present invention, the objects of pure black drawn line and low gloss cannot be achieved. It has been confirmed that when the balance is talc and / or mica, when at least 50% by weight of boron nitride is added, substantially the same effect as when only boron nitride is used is obtained. Boron nitride is 50
If the amount is less than 10% by weight, the properties of talc and / or mica appear, the bending strength is not exhibited, and the pencil lead has poor writing quality.

[0018]

EXAMPLES The present invention will be described below in more detail with reference to examples, but the present invention is not limited to the examples. (Example 1) 50 parts by weight of boron nitride powder having a graphitization index of 1.3 and an average particle diameter of 2.0 μm Vinyl chloride resin 50 parts by weight Dioctyl phthalate 15 parts by weight Zinc stearate 1 part by weight Knead and extrude into fine wire,
After heat-drying and infusibilizing treatment in air at 200 ° C., the fired body obtained by heating and firing to about 1,000 ° C. in a nitrogen atmosphere is impregnated with spindle oil to form a pencil lead with a diameter of 0.57 mm. Obtained. Table 1 shows the results of the bending strength test and the writing property test performed on the obtained pencil lead.

Example 2 A pencil lead having a diameter of 0.57 mm was obtained in the same manner as in Example 1 except that boron nitride powder having a graphitization index of 1.3 and an average particle size of 1.0 μm was used. Table 1 shows the results of the bending strength test and the writing property test performed on the obtained pencil lead.

Example 3 A pencil lead having a diameter of 0.57 mm was obtained in the same manner as in Example 1, except that boron nitride powder having a graphitization index of 1.3 and an average particle size of 0.6 μm was used. Table 1 shows the results of the bending strength test and the writing property test performed on the obtained pencil lead.

Example 4 A pencil lead having a diameter of 0.57 mm was obtained in the same manner as in Example 1, except that a boron nitride powder having a graphitization index of 1.0 and an average particle size of 6.0 μm was used. Table 1 shows the results of the bending strength test and the writing property test performed on the obtained pencil lead.

Comparative Example 1 A pencil lead having a diameter of 0.57 mm was obtained in the same manner as in Example 1, except that boron nitride powder having a graphitization index of 1.0 and an average particle diameter of 10 μm was used. Table 1 shows the results of the bending strength test and the writing property test performed on the obtained pencil lead.

Comparative Example 2 A pencil lead having a diameter of 0.57 mm was obtained in the same manner as in Example 1 except that boron nitride powder having a graphitization index of 11.0 and an average particle size of 0.6 μm was used. Table 1 shows the results of the bending strength test and the writing property test performed on the obtained pencil lead.

Comparative Example 3 A pencil lead having a diameter of 0.57 mm was obtained in the same manner as in Example 1 except that boron nitride powder having a graphitization index of 1.0 and an average particle size of 8.5 μm was used. Table 1 shows the results of the bending strength test and the writing property test performed on the obtained pencil lead.

Comparative Example 4 A pencil lead having a diameter of 0.57 mm was obtained in the same manner as in Example 1, except that boron nitride powder having a graphitization index of 15.7 and an average particle diameter of 1.5 μm was used. Table 1 shows the results of the bending strength test and the writing property test performed on the obtained pencil lead.

(Comparative Example 5) A pencil lead having a diameter of 0.57 mm was obtained in the same manner as in Example 1 except that natural flake graphite powder having a graphitization index of 1.0 and an average particle diameter of 3.5 μm was used. .
Table 1 shows the results of the bending strength test and the writing property test performed on the obtained pencil lead.

[0027]

[Table 1]

The bending strength shown in Table 1 is JIS S 60.
It is the result of the three-point bending test performed according to 05. Glossiness is 3 cm by 3 cm on high-quality paper by hand painting repeatedly so that the paper surface is uniformly filled in the left-right direction and the up-down direction. A test piece made by Suga Test Instruments UGV-5D It is the result measured at an angle. The hue and lightness are the results of measurement of the test piece used for the glossiness test using a color computer SM-4 manufactured by Suga Test Instruments.

When boron nitride having low crystallinity is used as in Comparative Examples 2 and 4, the bending strength is obviously low. Further, Comparative Examples 1 and 3 have higher bending strength than Comparative Examples 2 and 4 due to high crystallinity, but the bending strength is still insufficient as compared with the core using the graphite of Comparative Example 5, It is understood that according to the present inventions 1 to 4, a bending strength comparable to that of the core using the graphite of Comparative Example 5 is exhibited.

Further, the results of the glossiness show that the cores using boron nitride (Examples 1 to 4 and Comparative Examples 1 to 4) have smaller glossiness than the cores using the graphite of Comparative Example 5. The reason for this is not clear, but it is considered to be due to the presence or absence of pi electrons in graphite and boron nitride. Regarding the hue, when boron nitride is used in the same manner, it is black, but a so-called bluish black that exhibits a bluish purple hue is obtained, whereas the core using graphite is reddish black.

[0031]

EFFECTS OF THE INVENTION According to the present invention, it is possible to obtain a drawing line having a bending strength which is conventionally obtained only by using graphite, and which has a low glossiness and exhibits a bluish black color, which is ideal for giving a smooth writing feeling. You can get a nice pencil lead.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Yuichi Washio 1-10 Hitomi, Matsuida-cho, Usui-gun, Gunma Prefecture Shin-Etsu Chemical Gakuin Co., Ltd.

Claims (3)

[Claims] 1. In a fired pencil lead using substantially boron nitride as a crystalline lubricant of an inorganic material, boron nitride is substantially represented by a ratio of peak areas of X-ray diffraction, and a graphitization index [{(100 ) + (101)} / (102)] is 10 or less, and a pencil lead made of boron nitride having an average particle size of less than 8.0 μm. 2. The content of the inorganic body material which is substantially boron nitride, when the balance is graphite, is 80% by weight or more of boron nitride, and when the balance is talc and / or mica,
The pencil lead according to claim 1, which has a boron nitride content of 50% by weight or more. 3. Boron nitride reacted at a temperature of 1,500 ° C. or more or boron nitride reacted at a temperature of less than 1,500 ° C. is again heat-treated at 1,500 ° C. or more and is an average. A method for producing a pencil lead, characterized in that when the particle size exceeds 8.0 μm, the obtained powder is pulverized to an average particle size less than 8.0 μm and used.