Magnetic migration display panel

Abstract: A magnetic migration display panel comprising a dispersion liquid layer composed of a magnetic particle, a dispersion medium composed of an organic solvent, a thickening agent composed of fatty acid his-amide with a hydroxyl group having a melting point of 120°C to 160°C, and if desired a coloring material, on a substrate.

Description: The present invention relates to a magnetic migration display panel capable of displaying a distinct record by magnetism and capable of then erasing the record. Conventional magnetic migration display panels for displaying by migrating magnetic particles utilizing magnetism include an enclosed space in a multi-cell structure formed between two substrates where a plastic dispersion liquid including magnetic particles, dispersion medium, thickening agent, and if desired, coloring material is enclosed, or includes a layer of microcapsules enclosing the plastic dispersion liquid formed on a substrate. In a magnetic migration display panel, when writing for example with a magnetic pen or a head with electromagnets, along the substrate on the front side, a written line is displayed based on the contrast of the color of the plastic dispersion liquid and the color of the magnetic particles. This contrast is produced by migration of the magnetic particles attracted by the magnetism of the magnetic pen or the electromagnet from the substrate on the back side. The magnetic migration display panel can generally display, for example, a written letter or pattern, by attracting the magnetic particles in a dispersion system to a recording magnet. However, such a panel has the following problems: (a) Display cannot be maintained for a long time due to sedimentation of the attracted magnetic particles because the specific gravity of the magnetic particles is greater than that of the dispersion system. (b) A written line cannot be displayed distinctly due to attraction of magnetic particles situated far from the magnet, because all magnetic particles experiencing magnetism from the magnet are attracted to the magnet. To overcome these problems, a yield value is set by adding a thickening agent to the plastic dispersion liquid whereby migration of magnetic particles occurs when a force more than the yield value is applied as described in Japanese Examined Patent Publication No. 57-27463 by the inventors of the present invention. In conventional magnetic migration display panels, there is a problem that, for example, a whisker-like projection or a broken line is generated in a written line after successive writing and erasing with a writing magnet and an erasing magnet, even though the initial written line was distinct, having no blur line and no broken line. Through intense study by the present inventors, it was found that these phenomena may occur due to a temporary fracture of a frail three-dimensional structure formed by the thickening agent in the dispersion medium. Therefore, the inventors determined that the original frail three-dimensional structure formed by the thickening agent in the dispersion medium should be quickly reformed after its fracture when the temporary fracture of the frail three-dimensional structure occurs, so as to obtain a magnetic migration display panel capable of displaying stable distinct written lines constantly. The frail three-dimensional structure formed by the thickening agent utilizing a network such as of hydrogen bonds is easily densified with passage of time and is accelerated by heating. Especially when using an organic thickening agent the nearer the melting point of the thickening agent is to the storage temperature, the greater is the swelling of the particles of the thickening agent whereby a firm network is generated. It is found that recording and erasing cannot be practically conducted in such a case. In fact, magnetic migration display panels are frequently disposed in a car or a transportation container exposed to summer sun's rays, where the temperature reaches 50°C or more. If the display panel is left at a high temperature of 40°C or more, the display and erasing performance suffers an adverse effect for the above reason. It is not desirable for merchandise that a difference is generated in performance depending on the ambient temperature of use or the place of use. The present invention relates to a magnetic migration display panel including a dispersion liquid layer comprising magnetic particles, a dispersion medium, a thickening agent, and optionally a coloring material, on a substrate. The dispersion medium is an organic solvent. The thickening agent comprises a fatty acid amide with a hydroxyl group having a melting point of 120°C to 160°C. The dispersion liquid has a positive yield value. The dispersion medium can be a nonpolar organic solvent. The thickening agent can be selected from an alkylene-bis-12-hydroxy stearic acid amide and/or a phenylene-bis-12-hydroxy stearic acid amide, and 1 to 10% by weight thereof can be mixed in the dispersion liquid. A dispersion liquid layer in a magnetic migration display panel of the present invention is obtained by mixing magnetic particles, a dispersion medium, the above-mentioned thickening agent, and a coloring material as required. As a thickening agent for use in the present invention, there is used one or more fatty acid amides with a hydroxyl group having a melting point of 120°C to 160°C. Any such amides can be used. For example, an alkylene-bis-12-hydroxy stearic acid amide and/or a phenylene-bis-12-hydroxy stearic acid amide are useful. For example, methylene-bis-12-hydroxy stearic acid amide, ethylene-bis-12-hydroxy stearic acid amide, butylene-bis-12-hydroxy stearic acid amide, hexamethylene-bis-12-hydroxy stearic acid amide and xylylene-bis-12-hydroxy stearic acid amide are useful. One to 10% by weight of a single amide or a combination of amides can be used in the dispersion liquid. Further, other inorganic or organic thickening agents may be used as auxiliary thickening agents. The fatty acid bis-amide with a hydroxyl group as the thickening agent has a melting point of 120°C to 160°C. As described above, the network structure formed from the dispersion liquid by the thickening agent is promoted by heating. Especially if the temperature is near to the melting point of the thickening agent, the particles in the thickening agent swell, thereby generating a firm network. Consequently, migration of the magnetic particles is restricted more than necessary, which may result in incapability of writing and erasing. Therefore, taking the temperature of the magnetic migration panel in service into consideration, not less than 120°C for the melting point of the thickening agent is effective in practice use. However, the effect that the thickening agent produces a colloidal gel structure by swelling and dispersing in the solvent requires heating at about the melting point. Therefore, an excessively high melting point is not desirable with a view to safety and workability. Therefore, more than 160°C for the melting point is not preferable for practical production. Any desired magnetic particles can be used in any desired amount. For example, as the magnetic particles, 10 to 30% by weight of oxide magnetic materials such as black magnetite, γ-hematite, chromium dioxide and ferrite and metal magnetic material such as alloys made of cobalt or nickel, as powder or lamina in the dispersion liquid can be used. The magnetic particles may be granulated prior to use for the purpose of adjusting size or shape. Applicable size is from micro to large, and applicable shape is for example, globular, columnar, mass or lamina, depending on the purpose of use. In using the magnetic particles, various coloring agents can be mixed with the magnetic particles and/or surfaces of the magnetic particles can be coated with various materials to change the original color of the particles. If a variation in size of magnetic particles exists, migration ability is also varied. Therefore, uniform size of the magnetic particles is preferable to achieve a distinct display. As the dispersion medium, any organic solvents are useful. Nonpolar solvents such as oils or aliphatic hydrocarbons and polar solvents such as glycol and alcohol can be used. Especially, aliphatic hydrocarbons such as isoparaffins are preferable. Optionally, any desired coloring material can be used. As the coloring material, a white pigment, dyestuff, or other pigment can be used. Preferably not more than 10%, more preferably not more than 3% of coloring material, based on the total weight of the dispersion, is added to the plastic dispersion liquid to improve the contrast of the plastic dispersion liquid and the magnetic particles, so as to result in a distinct display. An excess amount of the coloring material may cause a blurred display with the magnetic particles. According to one embodiment of the present invention, a magnetic migration display panel is manufactured by forming an enclosed space with a multiple cell structure on a substrate, filling a plastic dispersion liquid into the enclosed space, and affixing another substrate thereon. According to another embodiment of the present invention, a magnetic migration display panel is manufactured by filling a plastic dispersion liquid into an enclosed space made from a plurality of concave portions formed on a substrate and affixing another substrate thereon. The concave portions are formed on one or both of the substrates and the substrates bound together by an adhesive for forming the independent enclosed space therebetween. According to yet another embodiment of the present invention, a magnetic migration display panel is manufactured by applying microcapsules that enclose a plastic dispersion liquid and binding them to a substrate using a binder. When occasion demands, a protective layer may be formed on a surface of the microcapsule applying layer to prevent the microcapsules from fracture by friction stress. Example 1 (a) Producing Plastic Dispersion Liquid 2.5 parts by weight of BIS-AMIDE KH (Trademark)(methylene-bis-12-hydroxy stearic acid amide manufactured by Nippon Kasei Co., Ltd) was added to 80.0 parts by weight of ISOPER-M (Trademark) (isoparaffin solvent manufactured by Esso Kagaku Co., Ltd). The mixture was heated to dissolve the amide and was cooled to obtain a dispersion liquid of BIS-AMIDE KH. 82.5 parts by weight of the dispersion liquid and 1 part by weight of TIPAQUE CR-50 (Trademark) (titanium oxide manufactured by Ishihara Sangyo Co., Ltd) were mixed using T.K. HOMOMIXER (Trademark) (wet dispersing machine manufactured by Tokushu Kakogyo Co., Ltd) to obtain 83.5 parts by weight of a white dispersion liquid. 80 parts by weight of TODA COLOR KN-320 (Trademark) (Magnetite manufactured by Toda Kogyo Co., Ltd) and 50 parts by weight of EPOTOHTO YD-017 (Trademark)(solid epoxy resin manufactured by Tohto Kasei Co., Ltd) in 40% of methylethyl ketone solution were kneaded. The mixture was dried and pulverized to obtain 50 parts by weight of black magnetic particles having 20 to 120 µm diameter. 16.5 parts by weight of the magnetic particles and 83.5 parts by weight of the white dispersion liquid were mixed to produce 100 parts by weight of the plastic dispersion liquid of the present embodiment. The yield value thereof was 1,88 N/m 2 (18.8 dyne/cm 2) measured by a direct method using a type B viscometer. (b) Manufacturing a Magnetic Migration Display Panel Firstly, a multiple cell plate in a honeycomb structure of about 0.8 mm height in the shape of a rough complete hexagon measuring about 2 mm each side formed by 0.065 mm thickness vinyl chloride was adhered to a vinyl chloride sheet having a thickness of about 0.15 mm using a ethylene-vinyl acetate series adhesive agent as a face side transparent substrate to produce a display panel member. Secondly, the plastic dispersion liquid was filled into each enclosed space in the multiple cell structure and the multiple cell plate was sealed with about 0.08 mm of transparent vinyl chloride sheet using an epoxy series adhesive agent to manufacture a magnetic migration display panel (Embodiment A). Examples 2 to 8 (a) Producing Plastic Dispersion Liquids The plastic dispersion liquids of each embodiment were prepared using the compositions shown in Table 1, as in Example 1. The yield values of the plastic dispersion liquid in each embodiment were shown in Table 1. (b) Manufacturing Magnetic Migration Display Panels The magnetic migration display panels filled with each dispersion liquid were manufactured as in Example 1 (Embodiment A). Example 9 (a) Producing a Plastic Dispersion Liquid The plastic dispersion liquid was prepared as in Example 1. (b) Manufacturing a Magnetic Migration Display Panel Firstly, a plurality of successive concave portions in the shape of a rough complete hexagon with about 2 mm of each side having about 1.3 mm in depth were formed on a vinyl chloride sheet having a thickness of about 0.15 mm using a vacuum forming mold to form a rear side transparent substrate. Secondly, the plastic dispersion liquid was filled into each concave portion, and the rear side transparent substrate was sealed with about 0.08 mm thickness of transparent vinyl chloride sheet as a face side substrate using an epoxy series adhesive agent to manufacture a magnetic migration display panel (Embodiment B). Example 10 (a) Producing A Plastic Dispersion Liquid The plastic dispersion liquid was prepared using compositions shown in Table 1, as in Example 1. The yield value of the plastic dispersion liquid is shown in Table 1. (b) Manufacturing A Magnetic Migration Display Panel First, 20 g of the dispersion liquid was added to 60 g of 10% gelatine solution to stir so as to be about 500 µm in a drop diameter of the dispersion liquid. Second, 60 g of 10% gum arabic solution was added to further stir the liquid. Third, 300 g of hot water at 50°C was added to the liquid to be kept at 50°C in the system. Fourth, acetic acid was added to achieve pH 4.2. Fifth, the system was cooled gradually to 50°C. Sixth, 10 g of 25% glutaric aldehyde solution was added to harden the capsule film. Seventh, 10% sodium hydroxide solution was added to the liquid to achieve pH 10.5 in the system. Finally, the temperature of the system was increased to 50°C and maintained at this temperature for 1 hour, and then was cooled gradually to room temperature to produce a micro capsule of the plastic dispersion liquid. The microcapsule was dispersed in an aqueous binder, applied to a polyester film having a thickness of 50 µm, and then dried to form a micro capsule dispersion layer. Thus a µm thick polyester film applied with a 100 µm adhesive layer in advance was stacked thereon to manufacture a magnetic migration display panel (Embodiment C). Comparative Examples 1 to 3 (a) Producing Plastic Dispersion Liquids The plastic dispersion liquids were prepared using the compositions shown in Table 1, as in Example 1. The yield values of the plastic dispersion liquid are shown in Table 1. (b) Manufacturing Magnetic Migration Display Panels The magnetic migration display panels filled with each dispersion liquid were manufactured as in Example 1 (Embodiment A). Test Method and Evaluation Thereof The magnetic migration display panels according to the above examples and the above comparative examples were tested for the following items, where a permanent magnet (2x2x3 mm in size) equivalent to JIS C2502 MPB380 was used to write at 25 cm/sec writing speed, and an anisotropic gum magnet (NT-5M-1504 manufactured by MagX Co., Ltd) with one surface magnetized was used to erase. (1) Retentivity of Display After leaving the writing for 1 day at a standstill, written lines were visibly observed. O no magnetic particles in the written lines had migrated down, and the written lines were distinct. x magnetic particles in the written lines had migrated down, and the written lines were blurred or had disappeared. (2) Distinctness of Display The written lines were visibly observed. O Widths of the written lines were constant and were displayed distinctly. x Whisker-like projections or cuts occurred in the written lines. (3) Stability of Display After repeated writing and erasing 50 times, the written lines were visibly observed. O Widths of the written lines were constant and were displayed distinctly. x Whisker-like projections or cuts occurred in the written lines. (4) Display and erase abilities after maintaining a temperature. After maintaining a temperature of the magnetic migration display panel in each example and each comparative example, displaying and erasing were conducted. Vividness of the display and complete erasability of the display at a time were observed leaving the panel at 0°C, at 20°C, or 60°C respectively for one week. O The written lines were displayed distinctly, and were erased completely in one operation without leaving an afterimage. x The written line were not displayed distinctly, and were not erased completely in one operation leaving an afterimage. Evaluations of each test are shown in Table 2. According to the magnetic migration display panel of the present invention, a plastic dispersion liquid composed of fatty acid bis-amide with a hydroxyl group having a melting point of 120°C to 160°C is used as a thickening agent. Therefore, excellent stability after elapse of time is obtained and no change in the nature of the plastic dispersion liquid dependant on the temperature used is shown. Thereby, a remarkably enhanced performance of the display panel, such as in recordability, retentivity, distinctness and erasability in displaying is obtained constantly. Reference in made to Japanese Patent Application No. 8-225773.

Claims: A magnetic migration display panel comprising a dispersion liquid including magnetic particles, a dispersion medium, a thickening agent, and optionally a coloring material, on a substrate, wherein said dispersion medium comprises an organic solvent, wherein said thickening agent comprises a fatty acid amide with a hydroxyl group having a melting point of 120°C to 160°C, and wherein said dispersion liquid has a positive yield value. The magnetic migration display panel according to claim 1, wherein said organic solvent comprises a nonpolar organic solvent. The magnetic migration display panel according to claim 1 or claim 2, wherein said thickening agent is selected from one or more of alkylene-bis-12-hydroxy stearic acid amides and phenylene-bis-12-hydroxy stearic acid amides, and the dispersion liquid comprises 1 to 10% by weight of the thickening agent, based on the total weight of the dispersion liquid. The magnetic migration display panel according to any one of the preceding claims, wherein the dispersion liquid comprises 10 to 30% by weight of said magnetic particles, based on the total weight of the dispersion liquid. The magnetic migration display panel according to any one of the preceding claims, wherein said dispersion liquid is located between two substrates. The magnetic migration display panel according to claim 5, wherein said dispersion liquid is located in an enclosed space in a multiple cell structure formed between said two substrates, and wherein said yield value of said dispersion liquid is in the range from 0.1 to 5 N/m 2 (1 to 50 dyne/cm 2 ). The magnetic migration display panel according to any one of claims 1 to 5, wherein said dispersion liquid is located in an enclosed space formed by a plurality of concave portions on the substrate and another substrate that covers said concave portions and is adhered to the former substrate, and wherein said yield value of said dispersion liquid is in the range from 0.1 to 5 N/m 2 (1 to 50 dyne/cm 2 ). The magnetic migration display panel according to any one of claims 1 to 4, wherein said dispersion liquid is encapsulated and said encapsulated dispersion liquid is adhered to said substrate by a binder, and wherein said yield value of said dispersion liquid is in the range from 0.1 to 5 N/m 2 (1 to 50 dyne/cm 2 ).

Inventors: NOJIMA TERUAKI, TAZAKI HIROSHI, NOJIMA, TERUAKI, TAZAKI, HIROSHI
Applicants: PILOT KK, KABUSHIKI KAISHA PILOT
Application year: 1997
Main classification: B43L1
IPC codes: G09F9, B43L1