Pencil lead

Abstract

In a pencil lead manufactured through a baking step regardless of being a black lead or a colored lead, the mechanical strength of the pencil lead is further increased by coating a surface thereof with a resin. A pencil lead includes a baked lead body whose surface is covered by a covering layer made of resin that has a scratch hardness of HB or greater. The scratch hardness of the resin as a subject resin is defined as a lowest hardness index of a tester pencil that is able to form an indentation on a surface of a horizontal flat plate coated with a 5-μm thick film of the subject resin, when the surface is scratched by the tester pencil being pressed onto the surface at an angle of 45° with a load of 7.355 N.

Claims

1. A pencil lead comprising a baked lead body, wherein: a surface of the baked lead body is covered by a covering layer made of resin, and a scratch hardness of the resin constituting the covering layer is 3H or greater, the scratch hardness of the resin as a subject resin being defined as a lowest hardness index of a tester pencil that is able to form an indentation on a surface of a horizontal flat plate coated with a 5-μm thick film of the subject resin, when the surface is scratched by the tester pencil being pressed onto the surface at an angle of 45° with a load of 7.355 N, in a state in which a tip end of a lead of the tester pencil has been left in a cylindrical shape while its distal end has been shaved in a flat plane.

2. The pencil lead of claim 1, wherein the resin is selected from the group consisting of organic-inorganic hybrid resins, polyolefin resins, vinyl resins, furan resins, ABS resins, phenol resins, ketone resins, maleic acid resins, acrylic resins, urea resins, urethane resins, epoxy resins, silicone resins, imide resins, amide resins, and fluorine resins.

3. The pencil lead of claim 1, wherein the covering layer further comprises a coloring material.

4. The pencil lead of claim 1, wherein the covering layer further comprises an ultraviolet absorber.

5. The pencil lead of claim 1, wherein the covering layer further comprises an antistatic agent.

6. The pencil lead of claim 2, wherein the covering layer further comprises a coloring material.

7. The pencil lead of claim 2, wherein the covering layer further comprises an ultraviolet absorber.

8. The pencil lead of claim 2, wherein the covering layer further comprises an antistatic agent.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is the cross-section of a lead of Example 2 photographed by a scanning election microscope.

DESCRIPTION OF EMBODIMENTS

(2) (1) Boron Nitride Baked Lead Body

(3) Boron nitride powder and a resin to be a binder are mixed (a plasticizer can also be added as appropriate), and the mixed composition is kneaded. The kneaded mixture is extrusion molded into a thin linear shape. This molded product is heated and baked in an oxygen-free atmosphere such as in an inert gas, and a baked lead body of boron nitride and a binder carbon is formed. By heating and baking this baked lead body in an oxygen-containing atmosphere, the binder carbon is oxidized and eliminated, and a porous baked lead body of only boron nitride is obtained. This porous baked lead body is white.

(4) This white-colored porous baked lead body is immersed for a predetermined time in a liquid in which a coloring material and an organic solvent are mixed to make the coloring material impregnated in the pores. Thereafter, the porous baked lead body is dried, and the organic solvent is eliminated.

(5) Finally, this porous baked lead body is immersed in oil to make the oil impregnated in the pores, and a colored lead is completed. Note that any oil can be used provided that it is an oil that is generally used in impregnation into a baked lead body, such as liquid paraffin or the like.

(6) Then, the above-described baked lead body is immersed for a predetermined time in a liquid in which an organic-inorganic hybrid resin (in particular, a silica-organic hybrid resin) and an organic solvent are mixed, and thereafter, by drying and eliminating the organic solvent, the surface is coated by the resin. Note that the coating by the resin may be carried out as a step between the coloring material impregnating step and the oil impregnating step as described above.

(7) Here, depending on the diameter size at the time of extrusion molding of the above-described kneaded mixture, the colored lead can also be formed as a pencil lead for a wood shaft and a refill lead for a lead holder of a final diameter of around 2 to 3 mm. Further, the colored lead can also be formed as refill leads for mechanical pencils of various thicknesses whose final diameters are 0.3 mm or greater and 1 mm or less.

(8) Note that the baked lead body can also be formed as a white lead, without including any coloring material in the above-described porous baked lead body.

(9) (2) Graphite Baked Lead Body

(10) Graphite in a form of a powder or flakes and a resin to be a binder are mixed (a plasticizer can also be added as needed), and further dispersed and kneaded. The kneaded mixture is extrusion molded into a thin linear shape. This molded product is heated and baked in an oxygen-free atmosphere such as in an inert gas, and a baked lead body of graphite and a binder carbon is formed. This porous baked lead body is black. Then, this porous baked lead body is immersed in oil to make the oil impregnated in the pores, and a graphite lead is completed. Note that any oil can be used provided that it is an oil that is generally used in impregnation into a baked lead body, such as liquid paraffin or the like.

(11) Then, the above-described baked lead body is immersed for a predetermined time in a liquid in which an organic-inorganic hybrid resin (in particular, a silica-organic hybrid resin) and an organic solvent are mixed, and thereafter, by drying and eliminating the organic solvent, the surface is coated by the resin. Note that the coating by the resin may be carried out as a step between the coloring material impregnating step and oil impregnating step as described above.

(12) Here, depending on the diameter size at the time of extrusion molding the above-described kneaded mixture, the graphite lead can also be formed as a pencil lead for a wood shaft and a refill lead for a lead holder of a final diameter of around 2 to 3 mm. Further, the graphite lead can also be formed as refill leads for mechanical pencils of various thicknesses whose final diameters are 0.3 mm or greater and 1 mm or less. EXAMPLES

(1) BORON NITRIDE BAKED LEAD

(13) A composition of the following combination was kneaded by a pressurizing kneader and a two-roll kneader, and this kneaded mixture was extrusion molded into a thin linear shape by an extruder, and was baked for 5 hours at 1,000° C. in an inert gas (nitrogen gas) circumstance, and a baked lead (hereinafter called a “BN lead”) of boron nitride and a binder carbon was obtained.

(14) Boron nitride powder: 50 wt. %

(15) Polyvinylchloride resin (binder resin): 40 wt. %

(16) Dioctyl phthalate (plasticizer): 10 wt. %

(17) This BN lead was further baked for 12 hours at 700° C. in the atmosphere, and a baked lead body of only boron nitride was prepared. Silicate 40 (Colcoat) was impregnated therein, and, after drying, the lead was baked for 5 hours at 1,000° C. in an inert gas circumstance, and a white baked lead body was obtained.

(18) A composition formed from a coloring material and an organic solvent of the following combination was impregnated into this white baked lead body, and the lead body was dried for 12 hours at 80° C.

(19) Spilon red C—PH (dye coloring material, Hodogaya Chemical): 20 wt. %

(20) Isopropyl alcohol (organic solvent): 80 wt. %

(21) Into the pores obtained by this drying, α-olefin oil (SpectraSyn 4, ExxonMobil) was impregnated for 6 hours at 100° C., and a red mechanical pencil lead of a diameter of 0.564 mm and a length of 60 mm was obtained. This red BN lead was used as Examples 1 to 10 and Comparative Example 1 and 2 described hereinafter.

(2) GRAPHITE BAKED LEAD

(22) A composition of the following combination was mixed and dispersed by a Henschel mixer and kneaded by a pressurizing kneader and a two-roll kneader, and this kneaded mixture was extrusion molded into a thin linear shape by an extruder. Thereafter, the plasticizer was dried and eliminated, and further, by carrying out a baking treatment for 10 hours at 1,000° C. in an inert gas (nitrogen gas) circumstance, a graphite baked lead (hereinafter called a “graphite lead”) of graphite and a binder carbon was obtained.

(23) Natural graphite A flakes: 40 wt. %

(24) Polyvinylchloride resin (binder resin): 40 wt. %

(25) Sodium stearate (surfactant): 1 wt. %

(26) Dioctyl phthalate (plasticizer): 19 wt. %

(27) Note that the above-listed natural graphite A flakes had an ab-surface of a flatness of 0.2 μm, an mv-value of 8 μm, a thickness of the c-axis of 1 μm, and an aspect ratio of 8. Into this lead body, α-olefin oil (SpectraSyn 4, ExxonMobil) was impregnated for 6 hours at 100° C., and a black mechanical pencil lead of a diameter of 0.564 mm and a length of 60 mm was obtained. This graphite lead was used in Example 11 and Comparative Example 3 described hereinafter.

(3) EXAMPLES

(3-1) Example 1

(28) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(29) Organic-inorganic hybrid resin (COMPOCERAN E203, silica-epoxy hybrid resin, Arakawa Chemical Industries): 15 wt. %

(30) Organic solvent (isopropyl alcohol): 85 wt. %

(31) Note that the scratch hardness of the coated film of a film thickness of 5 μm formed by the above-described resin solution was 8H.

(3-2) Example 2

(32) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface. Note that the cross-section of the lead of present Example 2 as photographed by a scanning electron microscope is FIG. 1. In the figure, a BN lead 10 portion of a blackish hue appears as if thin layers are superposed like a mille-feuille pastry, and it can be seen that a covering layer 20 of a whitish hue is formed on the surface layer thereof. Referring to the scale at the lower right of the drawing, the thickness of the covering layer 20 is approximately 1.0 μm, although there are fluctuations depending on the region. By the way, cross-sectional images that were about such as that of this drawing are confirmed in the other Examples as well.

(33) Organic-inorganic hybrid resin (COMPOCERAN E203, silica-epoxy hybrid resin, Arakawa Chemical Industries): 15 wt. %

(34) Pigment coloring material (MHI red, Mikuni Pigment): 60 wt. %

(35) Organic solvent (isopropyl alcohol): 25 wt. %

(36) Note that the scratch hardness of the coated film of a film thickness of 5 μm formed by the above-described resin solution was 8H.

(3-3) Example 3

(37) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(38) Organic-inorganic hybrid resin (COMPOCERAN E203, silica-epoxy hybrid resin, Arakawa Chemical Industries): 15 wt. %

(39) Ultraviolet absorber (HALSHYBRID UV-G Nippon Shokubai): 3 wt. %

(40) Organic solvent (isopropyl alcohol): 82 wt. %

(41) Note that the scratch hardness of the coated film of a film thickness of 5 μm that was formed by the above-described resin solution was 8H.

(3-4) Example 4

(42) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(43) Organic-inorganic hybrid resin (COMPOCERAN E203, silica-epoxy hybrid resin, Arakawa Chemical Industries): 15 wt. %

(44) Pigment coloring material (MHI red, Mikuni Pigment): 60 wt. %

(45) Antistatic agent (Haisera, Miyazaki Chemical): 3 wt. %

(46) Organic solvent (isopropyl alcohol): 22 wt. %

(47) Note that the scratch hardness of the coated film of a film thickness of 5 μm that was formed by the above-described resin solution was 8H.

(3-5) Example 5

(48) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(49) Epoxy resin (825, Mitsubishi Chemical): 15 wt. %

(50) Organic solvent (isopropyl alcohol): 85 wt. %

(51) Note that the scratch hardness of the coated film of a film thickness of 5 μm that was formed by the above-described resin solution was 3H.

(3-6) Example 6

(52) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(53) Ketone resin (K-90, Arakawa Chemical Industries): 15 wt. %

(54) Organic solvent (isopropyl alcohol): 85 wt. %

(55) Note that the scratch hardness of the coated film of a film thickness of 5 μm that was formed by the above-described resin solution was 3H.

(3-7) Example 7

(56) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(57) Urea resin (Fleamin, Daiwa Co., Ltd.): 15 wt. %

(58) Organic solvent (isopropyl alcohol): 85 wt. %

(59) Note that the scratch hardness of the coated film of a film thickness of 5 μm that was formed by the above-described resin solution was 3H.

(3-8) Example 8

(60) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(61) Fluorine resin (FG-5040, Fluoro Technology Co., Ltd.): 15 wt. %

(62) Organic solvent (isopropyl alcohol): 85 wt. %

(63) Note that the scratch hardness of the coated film of a film thickness of 5 μm that was formed by the above-described resin solution was 3H.

(3-9) Example 9

(64) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(65) Silicone resin (MRS-102, Yoshida KSK): 15 wt. %

(66) Organic solvent (isopropyl alcohol): 85 wt. %

(67) Note that the scratch hardness of the coated film of a film thickness of 5 μm that was formed by the above-described resin solution was 3H.

(3-10) Example 10

(68) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(69) Acrylic resin (A-1980XB, DIC): 15 wt. %

(70) Organic solvent (isopropyl alcohol): 85 wt. %

(71) Note that the scratch hardness of the coated film of a film thickness of 5 μm that was formed by the above-described resin solution was 3H.

(3-11) Example 11

(72) After the above-described graphite lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(73) Organic-inorganic hybrid resin (COMPOCERAN E203, silica-epoxy hybrid resin, Arakawa Chemical Industries): 15 wt. %

(74) Pigment coloring material (MHI red, Mikuni Pigment): 60 wt. %

(75) Organic solvent (isopropyl alcohol): 25 wt. %

(76) Note that the scratch hardness of the coated film of a film thickness of 5 μm that was formed by the above-described resin solution was 8H.

(4) COMPARATIVE EXAMPLES

(4-1) Comparative Example 1

(77) The above-described BN lead was used as is.

(4-2) Comparative Example 2

(78) After the above-described BN lead was immersed in a resin solution of the following composition, it was removed, and the solvent was dried, and a covering layer of a film thickness of around 1 μm was formed on the surface.

(79) Silicone resin (805 resin, Toray/Dow Corning): 15 wt. %

(80) Organic solvent (isopropyl alcohol): 85 wt. %

(81) Note that the scratch hardness of the coated film of a film thickness of 5 μm that was formed by the above-described resin solution was B.

(4-3) Comparative Example 3

(82) The above-described graphite lead was used as is.

(5) TESTING METHODS

(5-1) Strength Test

(83) The bending strength (unit: MPa) was measured for one hundred samples in accordance with JIS S 6005:2007, and the average was determined.

(5-2) Chuck Holding Force Test

(84) In a state of being set in a mechanical pencil, the lead was let out, and, in a state of being held by a chuck, the mechanical pencil was set perpendicular to a writing surface, and perpendicularly downward force was applied, and the force (unit: N) at the time when lead slip-through (which means that the lead was pushed and sent back in) occurred was measured. This measurement was carried out immediately after the lead was set, and after being knocked 10,000 times in the state in which the lead was set, respectively.

(5-3) Light-Fastness Test

(85) The lower half of the lead was covered in an aluminum foil so as to not be exposed, and the lead was set in a light-fastness testing fade meter (manufactured by Suga Testing Instruments, X25). After being exposed for 100 hours, it was visually confirmed whether or not the exposed portion had faded and the hue thereof had changed as compared with the portion covered by the aluminum foil. The evaluation standard was “A” in a case in which there was hardly any change in color, and “B” for a case in which fading occurred and a change in color was seen.

(5-4) Charging Performance Test

(86) In a state in which twenty leads were placed in a case, the leads were left for one day in a room of a temperature of 25° C. and a humidity of 10%. Thereafter, the case was shaken for one minute at a speed of around 2 times per second. The cover of the case was then opened, and the case was tilted such that the cover side thereof faced directly downward, and the number of leads that fell out was counted. The evaluation standard was “A” in a case in which the number of leads that fell out was 10 or more, and “B” in a case in which the number was 9 or less.

(6) TEST RESULTS

(87) The respective test results of above (5) are as per following Table 1. Note that the “-” in the table means, for the type of resin and scratch hardness, that coating by a resin was not carried out, and means, for light-fastness and charging performance, that testing was not carried out.

(88) TABLE-US-00001 TABLE 1 Chuck holding Contained components force (N) Example/ Anti- Ultra- After Comparative Scratch Coloring static violet Strength Immediately knocking Light- Charging example Lead type Resin type hardness material agent absorber (MPa) after setting 10,000 times fastness performance Example 1 BN Organic- 8H No No No 170 25.6 18.6 B B inorganic Example 2 BN Organic- 8H Yes No No 169 27.8 19.1 A B inorganic Example 3 BN Organic- 8H No No Yes 168 26.2 19.2 A B inorganic Example 4 BN Organic- 8H Yes Yes No 165 27.9 20.1 A A inorganic Example 5 BN Epoxy resin 3H No No No 155 27.2 20.2 B B Example 6 BN Ketone resin 3H No No No 152 26.9 17.9 B B Example 7 BN Urea resin 3H No No No 156 27.8 20.8 B B Example 8 BN Fluorine resin 3H No No No 154 23.4 16.4 B B Example 9 BN Silicone resin 3H No No No 154 23.7 14.7 B B Example 10 BN Acrylic resin 3H No No No 145 26.5 19.5 B B Example 11 Graphite Organic- 8H Yes No No 420 27.1 18.1 — — inorganic Comparative BN — — No No No 135 19.1 15.7 B B example 1 Comparative BN Silicone resin B No No No 138 23.5 19.5 B B example 2 Comparative Graphite — — No No No 398 18.8 14.8 — — example 3

(6-1) BN Lead

(89) As to the BN lead, each of Examples 1 to 10 in which the covering layer was formed had an excellent bending strength and an excellent chuck holding force as compared with Comparative Example 1 in which the covering layer was not formed.

(90) Moreover, each of Examples 1 to 10, in which the covering layer was formed by a resin whose scratch hardness was HB or greater, had an excellent bending strength as compared with Comparative Example 2 in which the covering layer was formed by a resin of a different product having a scratch hardness lower than HB.

(91) Further, among Examples 1 to 10, each of Examples 1 to 4 in which the resin had a relatively high scratch hardness of 8H had an excellent bending strength as compared with each of Examples 5 to 10 in which the resin had a relatively low scratch hardness of 3H.

(92) Note that each of Examples 2 and 4, in which a pigment coloring material was added to the covering layer, and Example 3, in which an ultraviolet absorber was added, had an excellent light-fastness as compared with each of Examples 1 and 5 to 10, in which neither of the pigment coloring material nor the ultraviolet absorber was added, thereby it can be expected that chronological color fading the lead color will be prevented.

(93) Further, from the results of Example 4, a static electricity preventing effect also could be expected due to the addition of an antistatic agent to the covering layer.

(6-2) Graphite Lead

(94) As to the graphite lead, Example 11 in which the covering layer was formed had an excellent bending strength and chuck holding force in as compared with Comparative Example 3 in which a covering layer is not formed. Note that, although not directly related to light-fastness, in Example 11, a red tint that is different from the black color of the lead could be expressed on the lead surface by adding the coloring material, thereby the design could be improved.

INDUSTRIAL APPLICABILITY

(95) The present invention can be used as a pencil lead or a mechanical pencil lead.