Baked pencil lead and method for manufacturing the same

Abstract: Baked pencil leads are disclosed herein which are prepared by kneading, extruding and baking a composition containing a graphite as a filler and a binder, wherein a degree of orientation π of the graphite in the direction of an extrusion axis measured by X-ray diffraction is 0.85 or more and the resultant leads have a flexural modulus of 100 GPa or more and a bending strength of 320 MPa or more. The baked pencil leads can be manufactured by carrying out the extrusion so that back face pressure just above an extrusion nozzle may be 300 kg/cm² or more and a shear rate at a time when the molten kneaded composition passes through the nozzle may be 5 × 10⁴ (1/sec) or more.

Description: The present invention relates to a baked pencil lead and a method for manufacturing the baked pencil lead. More specifically, it relates to a baked pencil lead manufactured by blending a graphite which is a filler and a binder as main components with a pore forming agent, kneading and extruding the blended components, and then subjecting it to a heat treatment at a high temperature; and a method for manufacturing the baked pencil lead. The graphite in the baked pencil lead obtained is highly oriented in the direction of an extrusion axis, and the baked pencil lead has well balanced strength, density and writing performance. Conventional wood-cased pencil leads and mechanical pencil leads have been manufactured by adding a binder such as a clay, a natural polymer, a synthetic polymer, a pitch and an asphalt to a colorant such as a graphite and carbon black, further, if necessary, adding a suitable solvent and plasticizer, kneading the composition, extruding the kneaded composition into a linear shape, baking it at a temperature of 900 to 1,200°C, and then impregnating the pores of the obtained baked lead with an oil. Of the conventional baked pencil leads, particularly the mechanical pencil leads having a small diameter are poor in strength, and for this reason, these leads do not have the practical strength yet, depending upon hardness. In order to improve the strength of the leads, various methods have been suggested. For example, Japanese Patent Application Laid-open No. Sho 62-64876 has suggested a method which comprises extracting and removing a plasticiser or a solvent from a kneaded composition containing the plasticizer or solvent, a colorant and a binder, and then extruding the composition. In Japanese Patent Application Laid-open No. Sho 62-129370, as a method for enhancing the strength without impairing a writing performance, there is suggested a method for manufacturing a pencil lead which comprises blending a graphite whisker thereto. Furthermore, Japanese Patent Application Laid-open No. Sho 63-156873 has suggested mechanical pencil leads having a constant density, less breakable properties and a smooth writing effect, which are manufactured by adding an aluminate-based coupling agent thereto. According to confirmation tests of these methods, the strength can be improved in all the methods, but the density deteriorates, and the writing performance also tends to deteriorate. In addition, various methods have been suggested in which much attention is paid to the orientation state of a filler in the leads to improve it. For example, Japanese Patent Application Laid-open No. Sho 64-38297 has suggested organic baked pencil leads having a diameter of 1.8 mm or more in which Young's modulus (flexural modulus) is 1.3 × 10 11 dyn/cm 2 (13 GPa) or more. This patent is limited to the organic baked leads having a large diameter of 1.8 mm or more, and detailed reference is not made to the relation between the flexural modulus and the strength of the leads. Moreover, in the thick leads having a large diameter of 1.8 mm or more, such a high shear rate as in the present invention can scarcely be obtained. Japanese Patent Publication No. Hei 1-49750 has suggested low-loading leads in which boron nitride is mainly used as a lubricant and a ratio of the peak height of (002) plane to the peak height of (100) plane of the hexagonal system boron nitride is 40 or less in X-ray diffraction obtained by irradiating the leads with X-ray vertically to the extrusion axis of the leads. This patent is limited to the leads mainly containing the boron nitride as the lubricant. In addition, according to information obtained from the X-ray diffraction and the detailed description of the suggested invention, it is apparent that the plate boron nitride is arranged at random in the direction of an extrusion axis, so that the boron nitride arranged vertically to the extrusion axis increases. Therefore, the disclosed patent is quite different from the present invention in conception. DE-A-4332582 discloses a method for controlling the orientation of fillers in the lead during the extrusion of the kneaded raw material. The resulting leads comprise tubular layers of the filling material, orientated concentrical around the lead axis. An object of the present invention is to provide baked pencil leads having well balanced strength, density and writing performance. Another object of the present invention is to provide a method for manufacturing the baked pencil lead. According to one aspect of the present invention, there is provided a baked pencil lead obtainable by the method comprising the steps of kneading a composition containing a graphite as a filler and a binder, extruding the kneaded composition, and baking the extruded product, wherein a degree of orientation π of the graphite in the direction of an extrusion axis measured by X-ray diffraction is 0.85 or more and the resultant baked pencil lead has a flexural modulus of 100 GPa or more and a bending strength of 320 MPa or more. According to another aspect of the present invention, there is provided a method for manufacturing a baked pencil lead comprising the steps of kneading a composition containing a graphite as a filler and a binder together with a pore forming agent, extruding the kneaded composition so that back face pressure just above an extrusion nozzle is 29.421 MPa (300 kg/cm 2 ) or more and a shear rate at a time when the molten kneaded composition passes through the extrusion nozzle is 5 × 10 4 (1/sec) or more, and baking the extruded product. It has been found that the strength of a lead is extremely closely correlated with the flexural modulus of the lead and the degree of orientation π of a graphite in the lead in the direction of an extrusion axis measured by X-ray diffraction. It is noted that the graphite crystals which are used as a filler have a plate-like and flat shape and its theoretical modulus is 1020 GPa which is much higher than those of other materials. Therefore, if the graphite is highly oriented in the direction of the extrusion axis in the baked pencil lead comprising the graphite and a carbon binder, the extremely high flexural modulus and strength can be obtained. In order to obtain the necessary and sufficient strength in the baked pencil lead, it is desirable that the degree of orientation π of the graphite in the lead in the direction of the extrusion axis is 0.85 or more, preferably 0.87 or more, and the flexural modulus of the lead is 100 GPa or more, preferably 110 GPa, whereby the lead having a strength of 320 MPa or more can be obtained. The above-mentioned degree of orientation π represents the degree of the orientation to the direction of the extrusion axis of the graphite crystals in the lead, and it is meant that the closer to 1 this value is, the higher the development of the orientation is. According to the X-ray diffraction, the X-ray is applied to the leads vertically to the direction of the extrusion axis of the leads, and an azimuth angle 2 is scanned from 0 to 1.5708 rad (90°) to confirm the position of the azimuth angle indicating the maximum value of an intensity distribution on (002) plane (about 0.4538 rad (26°)). Next, the leads are rotated 3.1416 rad (180°) in the vertical plane of an X-ray beam at the position of this azimuth angle, whereby the intensity distribution on the (002) plane is taken, and the degree of orientation π is calculated on the basis of a half width (H) at a point of 1/2 of the maximum intensity value in accordance with the following equation (1) Orientation degree π = (180 - H)/180. The flexural modulus of the leads is calculated in accordance with the following equation (2) Flexural modulus = 4PL 3 /3πd 4 δ (GPa) wherein P is a load at the measurement of bending strength, L is a distance between supports, d is a diameter of the leads, δ is a strain, and π is the ratio of the circumference of a circle to its diameter. On the basis of these knowledges, it has been paid much attention to extrusion conditions at the time of the extrusion molding as a concrete means for enhancing the degree of orientation π in the direction of an extrusion axis of the graphite in the leads, and as a result, it has been found that the degree of orientation π in the direction of the extrusion axis of the graphite in the leads and the flexural modulus can be enhanced by controlling back face pressure just above an extrusion nozzle at the time of extrusion molding and a shear rate at a time when a molten kneaded composition passes through the extrusion nozzle. In consequence, there can be obtained a baked pencil lead having well balanced strength, density and writing performance. Next, the method for manufacturing a baked pencil lead regarding the present invention will be described in detail. A binder such as a natural polymer, a synthetic polymer, a pitch and an asphalt is added to a graphite as a filler, and a pore forming agent is further added thereto. In addition, if necessary, a suitable solvent and plasticizer are added thereto. Then, they are kneaded, and the kneaded composition is then extruded into a linear shape. In this case, extrusion conditions are set so that back face pressure just above an extrusion nozzle is 29.421 MPa (300 kg/cm 2 ) or more, preferably 34.3245 MPa (350 kg/cm 2 ) or more and so that a shear rate at a time when the molten kneaded composition passes through the extrusion nozzle is 5 × 10 4 (1/sec) or more, preferably 7 × 10 4 (1/sec). The control of the extrusion conditions can be achieved by not only suitably selecting the materials to be used, regulating their blend, but also suitably combining a screw shape of an extruder, an extrusion temperature, an extrusion rate, a nozzle diameter and the like. The shear rate at the time when the molten kneaded composition passes through the nozzle can be calculated in accordance with the following equation (3) Shear rate = 4Q/πR 3 (1/sec) wherein Q is a flow rate (cm 3 /sec), R is a radius of the nozzle (cm), and π is the ratio of the circumference of a circle to its diameter. If both the conditions of the back face pressure and the shear rate are not simultaneously met, the necessary and sufficient bending strength cannot be obtained. Even if the back face pressure just above the extrusion nozzle is 29.421 MPa (300 kg/cm 2 ) or more, the degree of orientation in the direction of an extrusion axis of the graphite in the lead cannot be sufficiently heightened in the case that the shear rate at the time when the molten kneaded composition passes through the nozzle is less than 5 × 10 4 (1/sec). In consequence, the sufficient strength cannot be obtained. Conversely, even if the shear rate is 5 × 10 4 (1/sec) or more, the sufficient strength cannot be obtained in the case that the back face pressure just above the extrusion nozzle is less than 29.421 MPa (300 kg/cm 2 ). Under these conditions, the degree of orientation it in the direction of the extrusion axis of the graphite in the lead and the flexural modulus of the lead is relatively high, but the sufficient strength cannot be obtained. This reason can be considered to be that the back face pressure just above the extrusion nozzle is low, and hence sufficient compression stress cannot be applied to the molten kneaded composition, so that the cross-sectional tissue of the leads is not closely compacted, with the result that the sufficient strength cannot be obtained. In addition, when one blend composition is extruded changing the diameter alone of the extrusion nozzle at the time of the extrusion molding, there is a tendency that the smaller the diameter of the nozzle is, the higher the strength is. This reason can be considered to be that the smaller the diameter of the nozzle is, the larger a ratio (a ram ratio) of the screw diameter to the nozzle diameter of the extruder is, so that the back face pressure just above the extrusion nozzle increases, and as also understood from the equation (3), the smaller the nozzle diameter is, the higher the shear rate at the time when the molten kneaded composition passes through the nozzle is. The linear product thus obtained by the extrusion molding is baked at a temperature of 900 to 1200°C, and the pores of the obtained baked product are then impregnated with an oil to obtain the pencil leads of the present invention. Examples of the filler which can be used in the present invention include natural graphites, artificial graphites and kish graphites, and they can be used singly or in a combination of two or more kinds thereof. The average particle diameter of the filler is preferably 50 µm or less, more preferably 1 to 10 µm. If the particle diameter of the filler is more than 50 µm, the sufficient strength may not be obtained and if it is less than 1 µm, the orientation of the filler may be poor,the necessary strength may not be exerted, and hardness merely increases inconveniently. In this connection, the filler which is used herein plays not only a role as a reinforcing agent but also a role as a colorant. Examples of the binder which can be used in the present invention include thermoplastic resins such as vinyl chloride resins, chlorinated vinyl chloride resins and polyvinyl alcohols, thermosetting resins such as furan resins, phenolic resins and epoxy resins, natural polymers such as lignins, celluloses and tragacanth gums, pitches such as petroleum asphalt, coal tar pitch, naphtha cracking pitch and dry distillation pitch of a synthetic resin or the like, and they can be used singly or in a combination of two or more thereof. For the purpose of improving the characteristics of the composition at its kneading which is carried out adding high shear strength and/or improving the characteristics of the composition at the extrusion molding, there may be added, if necessary, one or more of plasticisers or solvents such as dioctyl phthalate (DOP), dibutyl phthalate (DBP), tricresyl phosphate (TCP), dioctyl adipate (DOA), diaryl phthalate (DAP), propylene carbonate, alcohols, ketones and esters. The writing performance of the pencil leads depends subtly upon not only the smoothness of the graphite but also the pores present in the pencil leads. Thus, in addition to the formation of the pores by the decomposition of the binder, the plasticizer and the like, a pore forming agent may be blended with the filler, the binder and the like, whereby a porosity and a pore distribution can be artificially regulated. Typical examples of the pore forming agent include sublimable organic compounds such as anthraquinone, fumaric acid and isophthalic acid, and depolymerizable polymers such as polyethylene, polypropylene and polytetrafluoroethylene. One or more of these pore forming agents may be added, if necessary. EXAMPLES Next, the present invention will be described in more detail with reference to examples, but the scope of the present invention should not be limited to these examples at all. Examples 1 to 3, Comparative Example 1 Graphite (natural scaly graphite; average particle diameter = 7 µm) 50 parts by weight Polyvinyl chloride 50 parts by weight Stearate 3 parts by weight Dioctyl phthalate 20 parts by weight These components were mixed by a Henschel mixer, kneaded by a pressure kneader and a twin-roll mill, and then extruded into a linear shape by the use of a screw type extruder equipped with an extrusion nozzle having a nominal diameter of 0.5 mm at an extrusion temperature of 100°C, while the rotational frequency of a screw was changed as occasion demands so as to obtain four shear rates. Afterward, the extruded composition was subjected to a heat treatment at 180°C for 10 hours in air in order to remove the remaining plasticizer therefrom, and at this time, the composition was solidified. After a certain time, the solidified composition was heated up to 300°C at 10°C/hr and from 300°C to 1000°C at 30°C/hr in a nitrogen atmosphere, baked at 1000°C for 1 hour, and finally immersed in alpha olefin oligomer (trade name Lipolub 20, made by Lion Co., Ltd.) to cause oil immersion, thereby obtaining mechanical pencil leads having a diameter of 0.570 mm. Examples 4 to 6, Comparative Example 2 The same blend and the same procedure as in Examples 1 to 3 were used except that the blend was extruded into a linear shape by the use of a screw type extruder equipped with an extrusion nozzle having a nominal diameter of 0.3 mm at an extrusion temperature of 100°C, while the rotational frequency of a screw was changed as occasion demands so as to obtain four shear rates, thereby obtaining mechanical pencil leads having a diameter of 0.380 mm. Comparative Examples 3 to 6 The same blend and the same procedure as in Examples 1 to 3 were used except that the blend was extruded into a linear shape at an extrusion temperature of 125°C, while the rotational frequency of a screw was changed as occasion demands so as to obtain four shear rates, thereby obtaining mechanical pencil leads having a diameter of 0.570 mm. With regard to Examples 1 to 6 and Comparative Examples 1 to 6, back face pressures at the time of the extrusion molding were measured, and the shear rates were calculated in accordance with the above-mentioned equation (3). For the obtained mechanical pencil leads, flexural moduli were calculated in accordance with the above-mentioned equation (2), and the degree of orientation π were calculated in accordance with the above-mentioned equation (1) by virtue of X-ray diffraction. In addition, bending strengths (MPa), densities (D) and hardnesses were obtained in accordance with JIS S6005. The results are shown in Table 1. As is apparent from the results of the examples, according to the present invention, when extrusion conditions are controlled so that back face pressure just above an extrusion nozzle at the extrusion molding is 29.421 MPa (300 kg/cm 2 ) or more and so that a shear rate at a time when a molten kneaded composition passes through an extrusion nozzle is 5 × 10 4 (1/sec) or more, and in baked pencil leads obtained under such conditions, the degree of orientation π of a graphite in the direction of an extrusion axis measured by X-ray diffraction becomes 0.85 or more, and the flexural modulus and the bending strength of the leads become 100 GPa or more and 320 MPa or more, respectively. Thus, in the baked pencil leads of the present invention, the strength can be remarkably improved without lowering density, and strength, density and writing performance are suitably balanced.

Claims: A method for manufacturing a baked pencil lead, comprising the steps of: kneading a composition containing a graphite as a filler and a binder together with a pore forming agent; extruding the kneaded composition so that back face pressure just above an extrusion nozzle is 29.421 MPa (300 kg/cm 2 ) or more and a shear rate at a time when the molten kneaded composition passes through the extrusion nozzle is 5 × 10 4 (1/sec) or more; baking the extruded product and impregnating the baked product with oil. The method for manufacturing a baked pencil lead according to Claim 1, wherein the baking is carried out in a temperature range of 900 to 1200°C. The method for manufacturing a baked pencil lead according to Claim 1 or 2, wherein the pore forming agent is selected from sublimable organic compounds, depolymerizable polymers and mixtures thereof. The method for manufacturing a baked pencil lead according to any of Claims 1 to 3, wherein the graphite is selected from natural graphites, artificial graphites, kish graphites and mixtures thereof. The method for manufacturing a baked pencil lead according to any of Claims 1 to 4, wherein the average particle diameter of the graphite is 50 µm or less. The method for manufacturing a baked pencil lead according to any of Claims 1 to 5, wherein the binder is selected from thermoplastic resins, thermosetting resins, natural polymers, pitches and mixtures thereof. The method for manufacturing a baked pencil lead according to Claim 6, wherein the thermoplastic resin is selected from vinyl chloride resins, chlorinated vinyl chloride resins, polyvinyl alcohols and mixtures thereof. The method for manufacturing a baked pencil lead according to Claim 6, wherein the thermosetting resin is selected from furan resins, phenolic resins, epoxy resins and mixtures thereof. The method for manufacturing a baked pencil lead according to Claim 6, wherein the natural polymer is selected from lignins, celluloses, tragacanth gums and mixtures thereof. The method for manufacturing a baked pencil lead according to Claim 6, wherein the pitch is selected from petroleum asphalt, coal tar pitch, naphtha cracking pitch, dry distillation pitch and mixtures thereof. The method for manufacturing a baked pencil lead according to any of Claims 1 to 10, wherein the composition contains a plasticizer or a solvent selected from dioctyl phthalate, dibutyl phthalate, tricresyl phosphate, dioctyl adipate, diaryl phthalate, propylene carbonate, alcohols, ketones, esters and mixtures thereof. A baked pencil lead obtainable by the method according to any of Claims 1 to 11. A pencil lead according to claim 12, wherein a degree of orientation π of the graphite in the direction of an extrusion axis measured by X-ray diffraction is 0.85 or more and the resultant baked pencil lead has a flexural modulus of 100 GPa or more and a bending strength of 320 MPa or more.

Inventors: ODASHIMA HIDEO, ODASHIMA, HIDEO
Applicants: MITSUBISHI PENCIL CO, MITSUBISHI PENCIL KABUSHIKI KAISHA
Application year: 1995
Main classification: C09D13
IPC codes: B43K19, C09D13