JP2008030367A - Base material film for rewritable recording medium and rewritable recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the base material film for a rewritable recording medium, which is excellent in color developing properties and decolorizing properties by heat energy, in addition the balance of both of which is well, and in visibility of colored print when employed as the base material of the rewritable recording medium and further in antistatic properties, and a rewritable recording medium. <P>SOLUTION: In the base material film for the rewritable recording medium employed in the rewritable recording medium formed by laminating a rewritable print layer onto one side of the base material film, the base material film for the rewritable recording medium has an antistatic layer comprising an antistat and a binder resin on the surface opposite to one laminated with the rewritable print layer of the plastic film, its light transmittance of ≤15% and its thermal conductance of 0.037-0.080 W/mK and, in addition, the value of the surface resistivity of the antistatic layer (at 25°C and in 65% RH) is ≤1×10<SP>13</SP>Ω/m<SP>2</SP>. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a base film for a rewrite recording medium capable of forming and erasing an image by controlling thermal energy, and a rewrite recording medium using the same. More specifically, when used as a base material for rewritable recording media, it has excellent color development and decoloring properties due to thermal energy, a good balance between the two, excellent visibility of the printed color, and antistatic properties. The present invention relates to a base film for a rewritable recording medium having excellent properties and a rewritable recording medium formed by laminating a rewritable printing layer thereon.

  In recent years, reversible thermosensitive recording media (hereinafter referred to as rewrite recording media) capable of recording and erasing images have attracted attention. A typical example is a rewritable recording medium composed of a reversible developer which usually develops a colorless or light leuco dye and heats the leuco dye to develop a color and then re-heats it to erase the color.

As a base material for such a rewritable recording material, paper, non-woven fabric, woven fabric, synthetic resin film, synthetic resin laminated paper, synthetic paper, metal foil, glass and the like can be used according to the purpose. For use in a thermal printer or word processor that is widely used and has a thermal head, paper, synthetic resin film, synthetic resin laminated paper, synthetic paper, and the like are preferable. Above all, the polyethylene terephthalate film used in prepaid cards such as telephone cards and orange cards not only has high mechanical strength, but also has high smoothness, making it easy to make the reversible thermosensitive recording layer a uniform layer. The image can be sharpened. For example, a transparent polyester film or a white polyester film is used as a base material for a rewritable recording material (see, for example, Patent Documents 1 and 2).
JP 2000-263934 A JP 2003-11504 A

On the other hand, as a heat-sensitive recording medium using a leuco dye as a color former, means for interspersing a minute air layer in a heat-sensitive color developing layer, or a heat insulating layer containing an air layer, for example, between a substrate and a heat-sensitive color developing layer A method for imparting heat insulation to a heat-sensitive recording medium and improving heat-sensitive color development using a means for forming a film is disclosed (for example, see Patent Documents 3 and 4).
JP 2002-301871 A JP 2002-258754 A

In addition, a technique for improving color developability by using a heat-insulating film or synthetic paper having an apparent density of 0.6 to 0.9 g / cm ³ as a base film of a thermal recording medium is disclosed. (For example, see Patent Document 5).
JP 2005-81626 A

  In the case of the heat-sensitive recording medium disclosed in the above-mentioned patent document, it is only necessary to develop a color. Therefore, it is preferable to impart heat insulating properties from the viewpoint of improving the color developability. On the other hand, in the case of a rewritable recording medium, reversibility of coloring and decoloring is required. In the case of the rewritable recording medium, unlike the heat-sensitive recording medium, it is necessary to heat it to a colored state and then rapidly cool it to room temperature to fix the colored state.

  That is, the leuco dye and the developer are heated to a temperature higher than their melting, and both are mixed in the molten state to cause color development, and then rapidly cooled from this color development state to fix the color development state. Therefore, the fixation of the colored state is affected by the temperature drop rate from the molten state. For example, in the cooling, decoloring occurs in the process of lowering the temperature, and the density is relatively lower than the decolored state before the temperature increase or the colored state fixed by rapid cooling.

  On the other hand, the provision of heat insulation acts in the negative direction with respect to rapid cooling. Therefore, regarding the color developability in the rewritable recording medium, the provision of heat insulation does not lead to the manifestation of a unique effect on the color developability, and there is an optimum value. For example, in the methods disclosed in Patent Documents 3 and 4, the heat insulation is insufficient. On the contrary, the method disclosed in Patent Document 5 has a problem that the heat developability is excessive and acts as a negative factor for rapid cooling, so that it cannot be said that the color developability is the best unlike the thermal recording medium. have.

  On the other hand, with regard to decoloring, when the temperature of the colored state fixed by cooling by the above method is raised again, decoloring starts at a temperature lower than the color developing temperature. Therefore, in general, decoloring is performed by heating at a temperature lower than the color development temperature to remove the cooling. Therefore, imparting heat insulation is preferable from the viewpoint of improving decolorization as in the case of color development on a heat-sensitive recording medium.

  In addition, in the method in which a minute air layer is scattered in the heat-sensitive color developing layer, light scattering is caused by the minute air layer contained in the heat-sensitive color developing layer, so that the sharpness of the printed image is deteriorated. There is a case. Moreover, the method of forming a heat insulation layer between a base material and a thermosensitive coloring layer is economically disadvantageous because the cost for forming the heat insulation layer is added. Furthermore, the method of forming a heat insulating layer between the base material layer and the thermosensitive coloring layer is because the surface of the heat insulating layer formed on the base material has irregularities, so that after the heat insulating layer is formed, the surface is smoothed by calendering. The necessity of this is described, and it has the problem that it becomes further disadvantageous in terms of economy.

  In the case of using a plastic film as a base material in a rewrite recording medium, since the film is easily charged, static electricity is generated due to friction with a roll or a thermal head during printing in the printer, and static electricity such as poor running can be generated. There is a problem that a failure occurs, and the improvement is desired.

  The object of the present invention is to solve the problems in the prior art described above, and when used as a base material for a rewritable recording medium, it is excellent in color developability and decoloring by heat energy and has a good balance between the two. Another object of the present invention is to provide a base film for a rewrite recording medium that is excellent in visibility of colored prints and that has excellent antistatic properties.

The base film for a rewrite recording medium of the present invention capable of solving the above problems is a base film for a rewrite recording medium used for a rewrite recording medium in which a rewrite printing layer is laminated on one side of the base film. The base film for a rewrite recording medium has an antistatic layer composed of an antistatic agent and a binder resin on the surface opposite to the side on which the rewrite printing layer of the plastic film is laminated, and has a light transmittance. 15% or less, the thermal conductivity is 0.037 to 0.080 W / mK, and the surface resistivity (25 ° C., 65% RH) of the antistatic layer is 1 × 10 ¹³ Ω / □ or less. It is characterized by.

  The light transmittance of the substrate film is more preferably 13% or less, and further preferably 10% or less. When the light transmittance exceeds 15%, the concealing property of the base film for rewritable recording medium is lowered, and the color of the printed layer provided on the surface of the film is lowered and the sharpness of the printed image is lowered. Although the lower limit of the light transmittance is not limited, it is preferably 1% or more, more preferably 3% or more from the viewpoint of suppressing the deterioration of the mechanical properties of the film due to the inclusion of a large amount of an additive that decreases the light transmittance described later. preferable.

  The upper limit of the thermal conductivity of the base film for a rewritable recording medium of the present invention is more preferably 0.077 W / mK, and further preferably 0.074 W / mK. On the other hand, the lower limit is more preferably 0.039 W / mK and even more preferably 0.041 W / mK. When the thermal conductivity of the film satisfies the above range, the color development and decoloring properties are improved and the balance between the two is improved when printing is performed using the base material of the rewritable recording medium.

  When the thermal conductivity exceeds 0.080 W / mK, the heat insulating property is insufficient, so that the loss of heat energy given for coloring and decoloring of the dye contained in the printing layer becomes large. In addition, due to insufficient heat insulating properties, it is necessary to increase energy such as when the color developability and decolorability are lowered, or when the temperature of the print head is increased to give a certain color developability and decolorization. Therefore, it is not preferable.

  On the other hand, when the thermal conductivity of the film is less than 0.037 W / mK, the heat insulation effect becomes too large, and it becomes difficult to fix the colored state by rapid cooling. For example, since heat applied for color development during color development is difficult to escape, a part of the dye is decolored by the heat and color developability deteriorates. Moreover, the operability at the time of manufacture of a film, such as deterioration of film quality such as the mechanical properties of the film due to imparting such characteristics, and frequent breakage during film production, which makes stable production impossible at an industrial level, decreases.

In addition, when the surface resistivity (25 ° C., 65% RH) of the antistatic layer formed on the plastic film is controlled to 1 × 10 ¹³ Ω / □ or less, it is used as a base film for a rewrite recording medium. For example, it is possible to suppress the occurrence of an electrostatic failure such as a running failure caused by friction with a roll or a thermal head during printing running in a printer. The surface specific resistance value (25 ° C., 65% RH) is preferably small, but if it is less than 1 × 10 ⁶ Ω / □, there is practically no difference in the effect of the present invention.

  The antistatic agent contained in the antistatic layer includes: (a) an anionic polymer compound having at least one sulfonate group or phosphate group in the molecule; and (b) a cationic polymer compound containing a quaternary ammonium base. Or (c) polythiophene or a derivative thereof. By using such a polymeric antistatic agent, when producing a long base film and winding it into a roll, the antistatic agent is applied to the surface of the plastic film opposite to the antistatic layer. Metastasis can be suppressed.

  Further, as a binder resin constituting the antistatic layer, a polyester graft polymer obtained by graft polymerization of a polymerizable unsaturated monomer containing at least one carboxyl group residue on a polyester resin containing at least an ethylene glycol residue Is preferably used. Since this resin is cured by a self-crosslinking reaction during the heat treatment during the production of the base film, when producing a long base film and winding it into a roll, the plastic film on the side opposite to the antistatic layer Transfer of the antistatic agent to the surface can be suppressed.

  In addition, by forming a coating layer containing this resin as a constituent component on the surface opposite to the antistatic layer, that is, the surface on which the rewritable printing layer is laminated, adhesion with the rewritable printing layer, in particular moisture and heat resistance. Adhesion can be improved.

  Furthermore, a low molecular weight material such as a cyclic oligomer or an unreacted monomer contained in the plastic film is formed on the film surface by forming a coating layer comprising the above polyester-based graft resin as a constituent component on at least one surface of the plastic film. Precipitation can also be suppressed. The reason for this will be described below.

  For example, when polyethylene terephthalate resin is used as a constituent material of a plastic film, it contains about 1% by mass of low molecular weight components such as cyclic trimer amount. Therefore, in the processing step of laminating the rewritable printing layer, or when the obtained rewritable recording medium is heated for color development or decoloration, the low molecular weight component in the plastic film is transferred to the film surface by heat, It may be deposited on the film surface.

  When the present inventors have observed the surface of the film in detail, the low molecular weight material mainly composed of oligomers and monomers such as cyclic trimers deposited on the surface of the polyester film has a particulate form. I understood. Conventional quantification of low molecular weight materials uses a method in which the surface of a polyester film is washed or eluted with a solvent (for example, chloroform) that dissolves low molecular weight materials, and cyclic trimers dissolved in the solvent are quantified. I came. However, in this method, not only the surface of the film but also low molecular weight substances existing in the film are extracted, so that the correspondence with the practical characteristics is not good.

Therefore, the present inventors paid attention to the fact that surface precipitates exist as particles, and deposited them on the surface of the film during heat treatment using a method capable of quantifying the area occupied by these precipitated particles deposited on the surface with a microscope. It was found that low molecular weight products. That is, in the processing step of laminating the rewritable printing layer, or when the obtained rewritable recording medium is heated for color development or color erasing, adhesion between the layers due to low molecular weight substances deposited on the surface of the substrate film to prevent problems, such as sexual decline in advance, a "film when heat-treated at 170 ° C. 20 min, filling ratio of particles deposited on the surface of the substrate film is 0.008μm ^{2 /} μm 2 or less" It is preferable to design the base film so as to have the function as follows.

Rewrite recording medium base material film of the present invention, when the film was heat treated at 170 ° C. 20 min, controls at least occupied area ratio of particles deposited on the surface of the antistatic layer 0.008μm ^{2 /} μm 2 or less It is preferable to do. Filling ratio of particles deposited on the antistatic layer surface after the heat treatment is more preferably at 0.007μm ² / μm ² or less, more preferably 0.006μm ² / μm ² or less, particularly preferably 0.005 .mu.m ² / Μm ² or less. When the occupied area ratio of the particles deposited on the surface exceeds 0.008 μm ² / μm ² , the stability of the performance is insufficient when coloring and decoloring are repeated in a rewrite recording medium. There is a case.

  In the present invention, as a binder resin constituting the antistatic layer, a polyester in which a polymerizable unsaturated monomer containing at least one carboxyl group is graft-polymerized to a polyester resin containing at least an ethylene glycol residue By using a resin mainly composed of a graft copolymer, the deposition of the low molecular weight component on the surface of the antistatic layer can be suppressed, which is a preferred embodiment. Similarly, on the opposite surface of the antistatic layer (the surface on the side where the rewritable printing layer is laminated), it is also preferable to provide a coating layer comprising the above polyester-based graft polymer as a constituent component.

Further, as the plastic film, a void-containing polyester film having an apparent density of 0.91 to 1.2 g / cm ³ is preferable.

The upper limit of the apparent density of the void-containing polyester film, more preferably 1.18 g / cm ^3, further preferably 1.17 g / cm ^3. On the other hand, the lower limit of the apparent density of the film is more preferably 0.92 g / cm ³ , further preferably 0.94 g / cm ³ . When the apparent density of the film exceeds 1.20 g / cm ³ , the decrease in thermal conductivity is insufficient. On the other hand, when the apparent density is less than 0.91 g / cm ³ , the void content is increased and the heat insulation is increased. Furthermore, the characteristics of the polyester film tend to be impaired, for example, the strength of the film decreases, the waist becomes weak, and the handleability deteriorates.

  The void-containing polyester film has a laminated structure with B / A or B / A / B as a unit, the B layer is a polyester layer containing a polyester resin and particles, and the A layer is a polyester resin and the polyester resin. A void-containing polyester layer made of a composition in which an incompatible thermoplastic resin is mixed is preferable. The B layer is a layer on which a rewrite print layer is laminated when a rewrite recording medium is used. Furthermore, the B layer is a layer that does not substantially contain a cavity inside, and preferably has a three-dimensional center plane average surface roughness (SRa) of 0.07 to 0.30 μm.

  The upper limit of the three-dimensional center plane average surface roughness (SRa) of the B layer is preferably 0.30 μm, more preferably 0.28 μm, and further preferably 0.25 μm. On the other hand, the lower limit is preferably 0.07 μm, more preferably 0.08 μm, still more preferably 0.09 μm. When the three-dimensional center plane average surface roughness (SRa) of the surface exceeds 0.30 μm, when the rewrite printing layer is laminated on the surface, the surface of the printing layer becomes rough and the fineness of the printed image decreases. May occur. On the other hand, if SRa is less than 0.07 μm, the slipperiness of the base film for rewritable recording medium is lowered and the handling property of the film tends to deteriorate.

  In the rewritable recording medium of the present invention, a rewritable printing layer is laminated on the surface opposite to the antistatic layer of the substrate film for rewritable recording medium. Among these, as a plastic film, it has a laminated structure with B / A or B / A / B as a unit, and the B layer does not substantially contain a cavity inside, and the three-dimensional center plane average surface roughness (SRa ) Is 0.07 to 0.30 μm, and the A layer is composed of a void-containing polyester layer, and a rewrite printing layer is laminated on the surface of the B layer of the base film for a rewrite recording medium using the void-containing laminated polyester film. The rewritable recording medium is the most preferred embodiment.

The base film for rewritable recording medium of the present invention has excellent heat developability and decoloring property when used as the base material of the rewritable recording medium because the thermal conductivity of the film is in a specific range. In addition, since the surface resistivity (25 ° C., 65% RH) of the antistatic layer is controlled to 1 × 10 ¹³ Ω / □ or less, when used as a support for a rewrite recording medium, for example, in a printer There is an advantage that the occurrence of static electricity trouble such as running failure caused by friction with a roll or a thermal head during printing running is suppressed.

  Furthermore, since the balance between color developability and decolorization by thermal energy is excellent, it is not necessary to interspers with the fine air layers for imparting heat insulation to the print layer. In addition, since the minute air layer contained in the printing layer does not cause light scattering, there is an advantage that the sharpness of the printed image does not deteriorate.

  Furthermore, since the base film is excellent in light-shielding properties, the sharpness of the printed image when the rewrite printing layer is provided is improved.

  Further, there is no need for calendering used to improve the surface roughness caused by the heat insulation layer lamination or the heat insulation layer required by a conventionally known method, which is advantageous in terms of economy, and a substrate for a rewritable recording medium. Suitable as a film.

In addition, by using a void-containing polyester film having an apparent density of 0.91 to 1.2 g / cm ³ as a plastic film, an appropriate cushioning property can be imparted to the substrate film. In this case, there is an advantage that the fineness of printing is improved.

  Furthermore, the plastic film has a laminated structure, and a layer that does not substantially contain voids is placed on the side where the rewrite printing layer is laminated, and the surface roughness of the layer that does not substantially contain voids is specified. By setting it as this range, the surface roughness of the printing layer laminated | stacked can be suppressed. As a result, the fineness of the printed image is improved. Moreover, by setting it as this laminated structure, it becomes difficult to cleave the film surface at the side which laminates a rewrite printing layer. Therefore, when used as a base material for a rewritable recording medium, there is an advantage that the durability of the rewritable printing layer is further improved.

The base film for a rewrite recording medium of the present invention is used as a base material for a rewrite recording medium in which a rewrite printing layer is laminated on one side of the base film. And the base film for a rewrite recording medium of the present invention has an antistatic layer comprising an antistatic agent and a binder resin as constituent components on the surface opposite to the side on which the rewrite printing layer of the plastic film is laminated, The light transmittance is 15% or less, the thermal conductivity is 0.037 to 0.080 W / mK, and the surface resistivity (25 ° C., 65% RH) of the antistatic layer is 1 × 10 ¹³ Ω / □ or less. It is characterized by being. The material used for the base film for a rewrite recording medium of the present invention and the method for producing the film will be described in detail below.

(1) Plastic film Examples of the resin constituting the plastic film used for the base film for the rewritable recording medium of the present invention include polyester resins such as polyethylene terephthalate and polyethylene naphthalate, and polyolefin resins such as polypropylene, polyethylene and cyclic polyolefin. And cellulose derivative resins such as cellulose triacetate, styrene resins such as syndiotactic polystyrene, and engineering plastic resins such as polyphenylene sulfide and polyimide. Of these, polyester resins are preferred because of their excellent cost performance.

  As the plastic film, a stretched polyester film formed by stretching at least in a uniaxial direction and heat treatment is preferable, and a biaxially stretched polyester film is particularly preferable.

  The polyester includes aromatic dicarboxylic acids or esters thereof such as terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, and glycols such as ethylene glycol, diethylene glycol, 1,3-propanediol, 1,4-butanediol, and neopentyl glycol. Is a polyester produced by polycondensation. In addition to the direct weight method in which an aromatic dicarboxylic acid and a glycol are directly reacted, these polyesters can be transesterified by an alkyl ester of an aromatic dicarboxylic acid and a glycol and then subjected to a polycondensation, or an aromatic method. It can be produced by a method such as polycondensation of diglycol ester of dicarboxylic acid.

  Typical examples of the polyester include polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate. The polyester may be a homopolymer or a copolymer of a third component. Among these polyesters, a polyester having an ethylene terephthalate unit, a trimethylene terephthalate unit, or an ethylene-2,6-naphthalate unit of 70 mol% or more, preferably 80 mol% or more, more preferably 90 mol% or more is preferable. The polyester constituting each layer may be the same type or different types, but the same type is preferable from the viewpoint of curling suppression and economy.

  The film made of polyester is preferably a biaxially oriented polyester film from the viewpoint of mechanical strength and thermal dimensional stability.

  As a method of making the light transmittance of the plastic film 13% or less, a method of blending a pigment having a concealing property with a resin constituting the film is preferable. In the present invention, since the purpose of reducing the light transmittance of the film is to improve the sharpness of the printed image, it is preferably white. Therefore, the substrate film for a rewritable recording medium of the present invention preferably has a whiteness measured in accordance with JIS Z-8722 of 75 or more, more preferably 80 or more. In order to set the whiteness of the film to 75 or more, it is preferable to contain white pigments such as titanium oxide, calcium carbonate, zinc oxide, or barium sulfate, polymer beads blended with these, and the like in the film. A method using a fluorescent whitening agent in combination is also effective.

  It is preferable to contain 1-25 mass% of said white pigment with respect to the resin composition which comprises a film. If the content of the white pigment is excessively increased in order to further improve the light-shielding property, breakage occurs frequently during film production, making it difficult to perform stable production at an industrial level.

  Moreover, as a method of setting the thermal conductivity of the film of the present invention to 0.037 to 0.080 W / mK, a method of incorporating a cavity in the film and utilizing a heat insulating effect of air contained in the cavity is preferable. . For example, in order to increase the amount of cavities in the film, the method of increasing the number of cavities in the film and the method of increasing the cavities are used alone or in combination so that the thermal conductivity of the film falls within the above range. The conditions are set as appropriate within the range that does not deteriorate. As for the specific conditions, the appropriate range varies depending on the material used and the equipment capacity, so it is not possible to set a numerical range in general. Therefore, first, a preliminary experiment with large conditions is performed, the thermal conductivity of the obtained film is evaluated, and the conditions under which the thermal conductivity can stably satisfy the range of 0.037 to 0.080 W / mK should be selected. That's fine. The condition setting is not performed by trial and error, but is performed according to the following technical guidelines.

  For example, in the case of a void-containing polyester film, a mixture of a polyester resin and a thermoplastic resin that is incompatible with the polyester resin is melted and then extruded into a sheet to form an unstretched film. Is at least uniaxially stretched and then heat treated.

  In order to increase the number of cavities in the film, a method of increasing the amount of the thermoplastic resin incompatible with the polyester, a method of reducing the size of the dispersion of the incompatible resin by physical or chemical means Is preferred. The physical means includes, for example, a method in which an incompatible resin is dispersed using a static mixer during melt extrusion, and the chemical means includes a method in which PEG or polystyrene is used in combination. Further, the incompatible resin will be described later.

  Moreover, as a method of increasing the size of the cavity, a method of increasing the stretching stress at the time of stretching the film (for example, lowering the stretching temperature or increasing the stretching ratio) is suitable.

  As a method for containing voids in a plastic film, (1) a method in which a foaming agent is contained and foamed by heat during extrusion or film formation, or foamed by chemical decomposition, (2) carbon dioxide gas at the time of extrusion or after extrusion, etc. (3) A method of blending hollow fine particles, (4) Heat incompatible with the thermoplastic resin A in the thermoplastic resin A constituting the film A method of containing a plastic resin B and stretching it uniaxially or biaxially after melt extrusion, (5) adding organic or inorganic fine particles to the thermoplastic resin A, and stretching it uniaxially or biaxially after melt extrusion. The method etc. can be mentioned. Moreover, you may implement combining these methods.

  Among the above methods, the methods other than the above (3) can simultaneously impart cushioning properties to the film by containing the cavities. When the cushioning property of the film is given, for example, when printing is performed by a thermal head method, an effect that the fineness of printing is improved is also added.

  Among the above-described void-containing methods, the method (4), that is, the thermoplastic resin A constituting the film contains the thermoplastic resin B incompatible with the thermoplastic resin A, and the film is stretched. One preferred embodiment is a method of generating cavities by peeling the interface between the two resins. As thermoplastic resin A which comprises a film, a polyester resin is preferable.

  When polyethylene terephthalate or polyethylene-2,6-naphthalate is used as the polyester resin, as the thermoplastic resin B incompatible with the polyester resin, for example, polyolefin resins typified by polypropylene and polymethylpentene, various types Examples thereof include modified polyolefin resins, polystyrene resins, polyacrylic resins, polycarbonate resins, polysulfone resins, cellulose resins, polyphenylene ether resins, and phenoxy resins. Moreover, you may use together white pigments, such as a titanium oxide, barium sulfate, zinc sulfide, a calcium carbonate.

  When a void-containing polyester film is used as the plastic film, a polyester resin and a thermoplastic resin that is incompatible with the polyester resin are used as a material for forming the film. As the thermoplastic resin incompatible with the polyester resin, a single thermoplastic resin may be used, or a plurality of thermoplastic resins may be used in combination. The content of the thermoplastic resin that is incompatible with the polyester resin is preferably 3 to 20% by mass, more preferably 5 to 15% by mass with respect to the polyester resin. When the content of the thermoplastic resin incompatible with the polyester resin is less than 3% by mass with respect to the polyester resin forming the void-containing polyester layer, the void content formed inside the film is reduced. The decrease in conductivity tends to be insufficient. On the other hand, when the content of the incompatible thermoplastic resin exceeds 20% by mass with respect to the polyester resin, breakage tends to occur frequently in the film manufacturing process.

In the present invention, in order to set the apparent density of the void-containing polyester film in the range of 0.91 to 1.2 g / cm ³ , for example, the inclusion of a thermoplastic resin incompatible with the polyester resin shown below. Examples thereof include a method for adjusting the amount to an appropriate value and a method for adjusting the stretching temperature and the stretching ratio of the film.

  The void-containing polyester film has a laminated structure with B / A or B / A / B as a unit, the B layer is a polyester layer containing a polyester resin and particles, and the A layer is non-phased with the polyester resin and the polyester resin. It is preferably a laminated film which is a void-containing polyester layer made of a composition in which a soluble thermoplastic resin is mixed. Note that the B layer is a layer on which a rewrite print layer is laminated when a rewrite recording medium is used. Furthermore, it is preferable that the B layer contains substantially no cavity.

  Here, the layer substantially free of voids (B layer) means a layer having a void lamination number density of less than 0.0 pieces / μm. Note that the cavity stacking number density is obtained by dividing the number of layers of cavities existing in the thickness direction of the B layer by the thickness of the B layer.

The cavity stacking number density can be measured using the following method.
A scanning electron microscope is used for observing the cavity of the film cross section, and a split cross section that is parallel to the longitudinal stretching direction of the film and perpendicular to the film surface is observed at five different portions of the sample. The observation is performed at an appropriate magnification of 300 to 3000 times, and a photograph is taken so that the dispersion state of the cavities in the entire thickness of the film can be confirmed. Next, a straight line is drawn perpendicularly to the film surface at any location on the photographic image, and the number of cavities that intersect the straight line in the B layer is counted. The number of cavities is defined as the number of cavities in the thickness direction of the B layer (the number of stacked layers). Further, the thickness (μm) of the B layer is measured along this straight line, and the number of cavities stacked in the B layer is divided by the thickness of the B layer to obtain the cavity stack number density (pieces / μm). In addition, measurement is performed at five places per photograph, and an average value of a total of 25 places is obtained to obtain a cavity layer number density of the B layer.

  In the present invention, when the void-containing polyester film having the above-described laminated structure is used as the plastic film, cleavage of the film surface can be suppressed. For example, when a foaming agent or a cavity-expressing material (a thermoplastic resin incompatible with polyester) is contained in the surface layer B, a reduction in surface strength due to void destruction tends to occur during processing.

  2-30 micrometers is preferable and, as for the thickness of B layer, 3-25 micrometers is more preferable. When the thickness of the B layer is less than 2 μm, it becomes difficult to suppress cleavage on the surface of the film. On the other hand, when the thickness of B layer exceeds 30 micrometers, it becomes disadvantageous at the point of heat insulation provision. In addition, it is preferable that the thickness of the said B layer shall be 5-25% with respect to the film total thickness.

  In addition, the thickness of the base film for rewritable recording media of the present invention may be appropriately determined depending on the application and standard when used in the market within a total thickness of 25 to 300 μm.

  In the case where the rewritable printing layer is laminated only on one side of the base film, in order to achieve the object of the present invention, the lamination of the B layer is sufficient on only one side. In addition, it is a more preferable embodiment that the B layer is laminated on both surfaces of the A layer from the viewpoint of suppressing curling of the laminated film.

  As a method for laminating the B layer, for example, a method of laminating two films obtained by individually forming the B layer and the A layer with an adhesive or the like may be used. You may use the method of extending | stretching and heat-processing the film which laminated | stacked A layer to at least one direction. The latter coextrusion method is more economical and is a preferred embodiment.

  In order to control the three-dimensional center plane average surface roughness (SRa) of the B layer to 0.07 to 0.30 μm, the average particle diameter of inorganic and / or organic fine particles contained in the polyester resin constituting the B layer, The content and the thickness of the B layer are adjusted as appropriate. The particles to be contained in the layer B are preferably white pigments having a function of imparting concealability to the film, and examples include titanium oxide (anatase type, rutile type), barium sulfate, calcium carbonate, silica, and zinc sulfide. These particles may be used alone or in combination of two or more.

  In the present invention, when a void-containing polyester film is used as the plastic film, it is preferably produced by the following method.

  As a method for producing a void-containing polyester film, a polyester resin and a mixture made of a thermoplastic resin incompatible with the polyester resin are melted and extruded into a sheet to obtain an unstretched film. A general method of stretching and then heat-treating can be used. When a laminated film having a layer structure of B / A or B / A / B is used, at least two extruders are used, the polyester resin is melted in one extruder, and the polyester is used in the other extruder. A mixture of a resin and a thermoplastic resin that is incompatible with the polyester resin is melted, joined by a feed block, and coextruded into a sheet to produce an unstretched film. The thickness of each layer can be adjusted by the amount of molten resin discharged from the B layer and the A layer.

  The conditions for stretching and orienting an unstretched film are closely related to the physical properties of the film. Below, taking the most general sequential biaxial stretching method, particularly a method of stretching an unstretched film in the longitudinal direction and then in the width direction, the stretching and heat treatment conditions will be described.

  In the longitudinal stretching step, stretching is performed between two or many rolls having different peripheral speeds. As a heating means at this time, a method using a heating roll or a method using a non-contact heating method may be used, or they may be used in combination. Next, the uniaxially stretched film is introduced into a tenter and stretched 2.5 to 5 times in the width direction at a temperature of (Tm-10 ° C.) or lower. However, Tm means the melting point of polyester.

  Further, the biaxially stretched film is subjected to heat treatment as necessary. The heat treatment is preferably performed in a tenter, and is preferably performed in the range of (Tm-60 ° C.) to Tm.

  When the film for rewritable recording medium of the present invention is produced by a method of containing cavities, it has a problem that curl curl generation is larger than a white polyester film containing a white pigment or a transparent polyester film. In particular, when the film for rewritable recording medium of the present invention is thick, the occurrence of curling becomes remarkable.

  Because, in the production of a thick biaxially stretched film, the unstretched film becomes very thick first, so the cooling on the cooling roll is clearly different between the cooling surface and the opposite side. The resulting structure will be different on the front and back of the film. Furthermore, since it is a void-containing polyester film containing fine cavities inside, the size, shape and volume fraction of the cavities easily change over the thickness direction of the film, so the physical properties and structure of the film front and back are the same. Such films are extremely difficult to manufacture.

  Therefore, in the present invention, it is preferable that the curled value after heat-treating the laminated polyester film at 110 ° C. for 30 minutes in an unloaded state is 1 mm or less. The curl value after the heat treatment is more preferably 0.9 mm or less, and further preferably 0.8 mm or less. When the curl value exceeds 1 mm, the flatness as a rewrite recording medium is deteriorated, which is not preferable.

  As a technique for suppressing curling, (1) a method for suppressing the occurrence of curling by resisting internal strain by reducing the volume fraction of the cavities and suppressing the size of each cavity, and (2) film (3) In order to control the curl caused by the structural difference between the front and back of the film applied in each step starting from the difference in crystallinity in the film thickness direction due to the cooling difference during extrusion. In addition, a method of positively generating a structural difference between the front and back of the film and complementing the necessary structural difference to bring the curl value close to zero is preferable.

  Specifically, the degree of orientation of the film front and back is controlled independently by setting the temperature or heat quantity of the film front and back to different values in the stretching process and heat setting process such as longitudinal stretching and lateral stretching, and the structure of the film front and back By adopting conditions that balance the physical properties, zero curl film formation is realized. In particular, a method in which the temperature or the amount of heat on the front and back sides of the film is different at a stage where the film thickness during longitudinal stretching is thick is suitable.

  In addition, as a basic requirement for stable production in a state where the curl is low over the entire width, it is also important to form cavities with little change in the film thickness direction by a stretching formulation with little thickness unevenness.

  More specifically, for the longitudinal curl immediately after film formation, the structural difference between the front and back of the film during longitudinal stretching is controlled, and the lateral curl is controlled by controlling the structural difference between the front and back of the film during lateral stretching and heat setting. It is preferable to create an internal strain in the opposite direction and balance the internal strain due to the structural difference between the front and back of the film, which is inevitably generated, and suppress curling.

  Moreover, as a curling generation suppression method, it is also effective to make the thickness of the surface layer (B) and the other surface layer (B ′) of the laminated polyester film the same.

(2) Antistatic layer The base film for a rewritable recording medium of the present invention has an antistatic layer comprising an antistatic agent and a binder resin as constituents on the surface of the plastic film opposite to the side on which the rewritable printing layer is laminated. The surface specific resistance (25 ° C., 65% RH) of the antistatic layer is controlled to 1 × 10 ¹³ Ω / □ or less.

(I) Antistatic agent In the present invention, the antistatic agent is an indispensable material for controlling the surface resistivity (25 ° C., 65 RH%) within the above range, and various antistatic agents are known. Yes. However, in the present invention, as the antistatic agent, (a) an anionic polymer compound having at least one sulfonate group or phosphate group in the molecule, and (b) a cationic polymer containing a quaternary ammonium base. It is preferable to use a polymer antistatic agent selected from a compound or (c) polythiophene or a derivative thereof.

  Because, when using a low molecular weight anionic antistatic agent that is widely used as an antistatic agent, when the long plastic film is wound into a roll, the antistatic agent is an antistatic layer of the plastic film. May transfer to the opposite side. For example, when a rewrite print layer is laminated on the surface to which the antistatic agent is transferred, the adhesion of the rewrite print layer is lowered. In order to prevent such transfer of the antistatic agent, it is preferable to use an anionic polymer compound, a cationic polymer compound, or a π-electron conjugated conductive polymer as the antistatic agent.

(Anionic polymer compound)
The anionic polymer compound is preferably a polymer antistatic agent having at least one sulfonate group or phosphate group in the molecule. This polymer antistatic agent is characterized by a structure having a large number of highly hydrophilic sulfonic acid components and phosphoric acid components. These polymer antistatic agents may be used alone or in combination of two or more.

  Resins containing a sulfonate group component in the molecule include polystyrene sulfonic acid sodium salts, potassium salts, lithium salts, ammonium salts, homopolymers such as phosphonium salts, and acrylic monomers such as acrylic acid esters or methacrylic acid esters. And a copolymer of styrene sulfonic acid monomer and a copolymer of acrylamidomethylpropane sulfonic acid and unsaturated monomer. In the present invention, the sulfonate can be a mixture of a metal salt and an amine salt.

  Examples of the resin containing a phosphate group in the molecule include resins obtained by polymerizing or copolymerizing various phosmers (manufactured by Unichemical) which are unsaturated monomers containing a phosphate group.

  The weight average molecular weight of the antistatic agent is preferably in the range of 1,000 to 1,000,000, more preferably 5,000 to 1,000,000. If the weight average molecular weight is less than 1,000, the transfer of the antistatic agent described above is increased. On the other hand, if it exceeds 1,000,000, it is preferable for suppressing the transition, but uniform mixing with the modified resin becomes difficult, and the uniformity of the coating film tends to be lowered.

(Cationic polymer compound)
As the cationic polymer compound, a polymer compound containing a quaternary ammonium base is preferable. This refers to a polymer compound having a component containing a quaternary ammonium base in the main chain or side chain in the molecule. Examples of such constituents include, for example, a pyrrolidinium ring, an alkylamine quaternized product, a copolymer of these with acrylic acid or methacrylic acid, an N-alkylaminoacrylamide quaternized product, a vinylbenzyltrimethylammonium salt, 2-hydroxy 3-methacryloxypropyl trimethyl ammonium salt etc. are mentioned. Further, these may be combined or copolymerized with other resins. In addition, examples of anions that serve as counter ions of these quaternary ammonium salts include ions such as halogen, alkyl sulfate, alkyl sulfonate, and nitric acid.

(Π-electron conjugated conductive polymer)
Examples of the π-electron conjugated conductive polymer include polyaniline or a derivative thereof, polypyrrole / or a derivative thereof, polyisothianaphthene or a derivative thereof, polyacetylene or a derivative thereof, and polythiophene or a derivative thereof. Among them, polythiophene or a derivative thereof is particularly preferable from the viewpoint of little coloring.

(Ii) Binder resin In the present invention, a binder resin is used in the antistatic layer for the purpose of holding an antistatic agent. Examples of the binder resin include polyester, polyurethane, acrylic resin, vinyl resin, epoxy resin, amide resin, polyvinyl alcohol, or a copolymer of each resin. These resins may be resins grafted by introducing a copolymer component into the main chain of each resin. Examples of the graft resin include acrylic resin graft polyester, acrylic resin graft polyurethane, vinyl resin graft polyester, and vinyl resin graft polyurethane.

  In the present invention, among these graft resins, a polyester graft polymer obtained by graft polymerization of a polymerizable unsaturated monomer containing at least one carboxyl group residue on a polyester resin containing an ethylene glycol residue, It is particularly preferable as a binder resin constituting the antistatic layer.

  In this specification, the “polyester-based graft polymer” means that all the polyester-based resin molecules are graft-polymerized with the above acid anhydride, or the polyester-based resin has the above-mentioned acid. It means a mixture of graft polymer molecules grafted with anhydride and unreacted polyester resin molecules not grafted with acid anhydride as described above.

  The polyester-based graft polymer can form a cross-link between resin molecules by the action of the unit by introducing a unit derived from the above acid anhydride into the polyester-based resin. For example, when a coating solution containing a polyester-based graft polymer as a main component is prepared, the acid anhydride group in the resin molecule is changed to a carboxyl group by hydrolysis or the like in the coating solution. Thereafter, the coating solution is applied to a base film, and an acid anhydride group is formed between resin molecules or active hydrogen groups possessed by other molecules by a thermal history applied when forming a coating layer by drying or the like. A cross-linked structure is formed between resin molecules, for example, by pulling out and generating an ester group.

  The above “grafting” is to introduce a branched polymer composed of a polymer different from the main chain into the main chain of the main polymer.

  Graft polymerization is generally carried out by reacting at least one polymerizable unsaturated monomer using a radical initiator in a state where a hydrophobic copolymerizable polyester resin is dissolved in an organic solvent. . The reaction product after completion of the grafting reaction includes, in addition to a desired graft polymer of a hydrophobic copolymerizable polyester and a polymerizable unsaturated monomer, a hydrophobic copolymerizable polyester resin that has not undergone grafting, and It also contains a polymer of the above unsaturated monomer that has not been grafted to the hydrophobic copolymerizable polyester. The polyester-based graft polymer in the present invention includes not only the above-mentioned polyester-based graft polymer but also a reaction including an unreacted hydrophobic copolymerizable polyester, a polymer of an unsaturated monomer that has not been grafted, and the like. Also referred to as a mixture.

The acid value of the polyester-based graft polymer obtained by graft polymerization of at least one polymerizable unsaturated monomer to the hydrophobic copolymerizable polyester resin is preferably 600 eq / 10 ⁶ g or more. More preferably, it is 1200 eq / 10 ⁶ g or more. When the acid value of the graft polymer is less than 600 eq / 10 ⁶ g, it cannot be said that the adhesiveness with the layer coated with the graft polymer-containing layer which is the object of the present invention is sufficient.

  Moreover, the mass ratio of the hydrophobic copolymerizable polyester resin and the polymerizable unsaturated monomer to obtain a desired graft polymer is polyester / polymerizable unsaturated monomer = 40/60 to 95/5. The range is desirable, more desirably 55/45 to 93/7, and most desirably 60/40 to 90/10. When the mass ratio of the hydrophobic copolymerizable polyester resin is less than 40% by mass, the excellent adhesiveness of the polyester resin cannot be exhibited. On the other hand, when the mass ratio of the hydrophobic copolymerizable polyester resin is larger than 95% by mass, blocking, which is a defect of the polyester resin, easily occurs.

  The graft polymer is in the form of an organic solvent solution or dispersion, or an aqueous solvent solution or dispersion. In particular, a dispersion of an aqueous solvent, that is, a form of a water-dispersed resin is preferable from the viewpoint of working environment and applicability. Such a water-dispersed resin is usually obtained by graft polymerization of at least one hydrophilic polymerizable unsaturated monomer onto the hydrophobic copolymerizable polyester resin in an organic solvent, followed by water addition and organic It can be obtained by distilling off the solvent.

  The polyester-based graft polymer preferably exhibits a translucent or milky white appearance having an average particle diameter of 500 nm or less, particularly 10 to 500 nm, measured by a laser light scattering method. Graft polymers with various particle sizes can be obtained by adjusting the polymerization method. From the viewpoint of dispersion stability, the average particle size is preferably 400 nm or less, more preferably 300 nm or less. When it exceeds 500 nm, the gloss of the coating film surface is lowered and the transparency is lowered. On the other hand, if it is less than 10 nm, the water resistance, which is the object of the present invention, is lowered, which is not preferable.

  The polymerizable unsaturated monomer to be grafted to the hydrophobic copolymerizable polyester resin is a hydrophilic radical polymerizable monomer, and is a radical polymerization having a hydrophilic group or a group that can be changed to a hydrophilic group later. It is a possible monomer. Examples of the hydrophilic group include a carboxyl group, a hydroxyl group, a phosphoric acid group, a phosphorous acid group, a sulfonic acid group, an amide group, and a quaternary ammonium base. On the other hand, examples of the group that can be changed to a hydrophilic group include an acid anhydride group, a glycidyl group, and a chloro group. Among these groups, a carboxyl group is preferable from the viewpoint of increasing the water dispersibility and the acid value of the graft polymer. Therefore, a polymerizable unsaturated monomer having a carboxyl group or a group capable of becoming a carboxyl group is preferable.

  The glass transition temperature of the graft polymer is 30 ° C. or lower, preferably 10 ° C. or lower. By using a graft polymer having a glass transition temperature of 30 ° C. or lower for the graft polymer-containing layer, a laminated polyester film having excellent adhesion can be obtained. When the physical properties of the graft polymer are out of the above range, the effect of the graft polymer-containing layer containing the graft polymer is hardly exhibited.

  The above-mentioned hydrophobic copolymerizable polyester resin needs to be essentially water-insoluble which does not inherently disperse or dissolve in water. When a polyester resin that is dispersed or dissolved in water is used for graft polymerization, the adhesiveness and water resistance, which are the objects of the present invention, are deteriorated. The composition of the dicarboxylic acid component of this hydrophobic copolymerizable polyester resin is as follows: aromatic dicarboxylic acid 60 to 99.5 mol%, aliphatic dicarboxylic acid and / or alicyclic dicarboxylic acid 0 to 40 mol%, polymerizable unsaturated It is preferable that it is 0.5-10 mol% of dicarboxylic acid containing a double bond. When the aromatic dicarboxylic acid is less than 60 mol%, or when the aliphatic dicarboxylic acid and / or alicyclic dicarboxylic acid exceeds 40 mol%, the adhesive strength is lowered.

  Moreover, when the dicarboxylic acid containing a polymerizable unsaturated double bond is less than 0.5 mol%, it becomes difficult to efficiently graft the polymerizable unsaturated monomer to the hydrophobic copolymerizable polyester resin. On the other hand, if it exceeds 10 mol%, the viscosity increases too late in the grafting reaction, which is not preferable because it prevents the uniform progress of the reaction. More preferably, the aromatic dicarboxylic acid is 70 to 98 mol%, the aliphatic dicarboxylic acid and / or the alicyclic dicarboxylic acid is 0 to 30 mol%, and the dicarboxylic acid containing a polymerizable unsaturated double bond is 2 to 7 mol%. It is.

  Examples of the aromatic dicarboxylic acid include terephthalic acid, isophthalic acid, orthophthalic acid, naphthalenedicarboxylic acid, biphenyldicarboxylic acid and the like. It is preferable not to use a hydrophilic group-containing dicarboxylic acid such as 5-sodium sulfoisophthalic acid because the water resistance, which is the object of the present invention, decreases. Examples of the aliphatic dicarboxylic acid include succinic acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, and dimer acid. Examples of the alicyclic dicarboxylic acid include 1,4-cyclohexanedicarboxylic acid, 1, Examples include 3-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid and its acid anhydride. Examples of dicarboxylic acids containing polymerizable unsaturated double bonds include α, β-unsaturated dicarboxylic acids, fumaric acid, maleic acid, maleic anhydride, itaconic acid, citraconic acid, unsaturated double bonds Examples of the alicyclic dicarboxylic acid to be used include 2,5-norbornene dicarboxylic acid anhydride and tetrahydrophthalic anhydride. Of these, fumaric acid, maleic acid, and 2,5-norbornene dicarboxylic acid are preferred from the viewpoint of polymerizability.

  On the other hand, examples of the glycol component include aliphatic glycols having 2 to 10 carbon atoms and / or alicyclic glycols having 6 to 12 carbon atoms and / or ether bond-containing glycols. Examples of the aliphatic glycol having 2 to 10 carbon atoms include ethylene glycol, 1,2-propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6 -Hexanediol, 3-methyl-1,5-pentanediol, 1,9-nonanediol, 2-ethyl-2-butylpropanediol, and the like. Examples of the alicyclic glycol having 6 to 12 carbon atoms include 1,4-cyclohexanedimethanol. In the present invention, the glycol component preferably comprises ethylene glycol. Satisfying this requirement is a preferred embodiment in terms of cost performance of the polyester resin.

  Examples of the ether bond-containing glycol include diethylene glycol, triethylene glycol, dipropylene glycol, and glycols obtained by adding ethylene oxide or propylene oxide to two phenolic hydroxyl groups of bisphenols, such as 2,2-bis (4 -Hydroxyethoxyphenyl) propane and the like. Polyethylene glycol, polypropylene glycol, and polytetramethylene glycol may be used as necessary.

  In the hydrophobic copolymerizable polyester resin, 0 to 5 mol% of a trifunctional or higher polycarboxylic acid and / or polyol can be copolymerized. As the trifunctional or higher polycarboxylic acid, (anhydrous) Trimellitic acid, (anhydrous) pyromellitic acid, (anhydrous) benzophenone tetracarboxylic acid, trimesic acid, ethylene glycol bis (anhydro trimellitate), glycerol tris (anhydro trimellitate) and the like are used. On the other hand, glycerin, trimethylolethane, trimethylolpropane, pentaerythritol and the like are used as the trifunctional or higher functional polyol. The tri- or higher functional polycarboxylic acid and / or polyol is copolymerized in the range of 0 to 5 mol%, preferably 0 to 3 mol% with respect to the total acid component or the total glycol component. Gelation during polymerization tends to occur, which is not preferable. The weight average molecular weight of the hydrophobic copolymerizable polyester resin is preferably in the range of 5,000 to 50,000. When the weight average molecular weight is less than 5,000, the adhesive strength is lowered. On the other hand, when the weight average molecular weight exceeds 50,000, problems such as gelation at the time of polymerization occur.

The polymerizable unsaturated monomer includes, for example, fumaric acid, monoethyl fumarate, diethyl fumarate, diester of fumaric acid such as dibutyl fumarate; maleic acid and its anhydride, monoethyl maleate, maleic acid Monoester or diester of maleic acid such as diethyl and dibutyl maleate; Itaconic acid and its anhydride, monoester or diester of itaconic acid; maleimide such as phenylmaleimide; styrene, α-methylstyrene, t-butylstyrene, chloro Styrene derivatives such as methylstyrene; vinyltoluene, divinylbenzene and the like. Acrylic polymerizable monomers that are one of polymerizable unsaturated monomers include, for example, alkyl acrylate, alkyl methacrylate (the alkyl group is methyl, ethyl, n-propyl, isopropyl, n-butyl). Group, isobutyl group, t-butyl group, 2-ethylhexyl group, cyclohexyl group, phenyl group, benzyl group, phenylethyl group, etc.): 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2- Hydroxypropyl methacrylate-containing acrylic monomer: acrylamide, methacrylamide, N-methyl methacrylamide, N-methyl acrylamide, N-methylol acrylamide, N-methylol methacrylamide, N, N-dimethylol acrylamide N-methoxymethylacrylamide, N-methoxymethylmethacrylamide, amide group-containing acrylic monomer of N-phenylacrylamide: N, N-diethylaminoethyl acrylate, amino group-containing acrylic monomer of N, N-diethylaminoethyl methacrylate : Glycidyl acrylate, epoxy group-containing acrylic monomer of glycidyl methacrylate: Acrylic monomers containing carboxyl groups or salts thereof such as acrylic acid, methacrylic acid and their salts (sodium salt, potassium salt, ammonium salt) It is done. However, acrylic polymerizable monomers are less preferred for use in the present invention because they are less effective in reducing the surface energy hydrogen bond strength component term (γs ^h ) of the present invention. The said polymerizable unsaturated monomer can be copolymerized using 1 type, or 2 or more types. Among the above monomers, maleic anhydride and its ester, and a combination of maleic anhydride and styrene are preferable.

  As the graft polymerization initiator, organic peroxides and organic azo compounds known to those skilled in the art can be used. Benzoyl peroxide, t-butylperoxypivalate as organic peroxide, 2,2'-azobisisobutyronitrile, 2,2'-azobis (2,4-dimethylvaleronitrile) as organic azo compound Etc. The amount of the polymerization initiator used for the graft polymerization is at least 0.2% by mass, preferably 0.5% by mass or more, based on the polymerizable unsaturated monomer. In addition to the polymerization initiator, a chain transfer agent for adjusting the chain length of the branched polymer, for example, octyl mercaptan, mercaptoethanol, 3-t-butyl-4-hydroxyanisole and the like may be used as necessary. In this case, it is desirable to add in the range of 0 to 5% by mass with respect to the polymerizable unsaturated monomer.

(Iii) Crosslinking agent Further, by adding a crosslinking agent to the coating solution for the antistatic layer and causing a crosslinking reaction with the functional group of the binder resin, the binder resin is imparted with a high degree of water resistance or the opposite of the antistatic layer. The transfer to the surface can be suppressed. In particular, when used in combination with the above graft polymer, these effects become more remarkable.

  As a cross-linking agent, phenol formaldehyde resins of condensates with formaldehyde such as alkylated phenols and cresols; adducts of urea, melamine, benzoguanamine and the like with formaldehyde, these adducts and alcohols having 1 to 6 carbon atoms An amino resin such as an alkyl ether compound comprising: a polyfunctional epoxy compound; a polyfunctional isocyanate compound; a blocked isocyanate compound; a polyfunctional aziridine compound; an oxazoline compound, and the like. Examples of the phenol formaldehyde resin include alkylated (methyl, ethyl, propyl, isopropyl or butyl) phenol, p-tert-amylphenol, 4,4′-sec-butylidenephenol, p-tert-butylphenol, o-, m-, p-cresol, p-cyclohexylphenol, 4,4'-isopropylidenephenol, p-nonylphenol, p-octylphenol, 3-pentadecylphenol, phenol, phenyl o-cresol, p-phenylphenol, xylenol, etc. Mention may be made of condensates of phenols and formaldehyde.

  Examples of amino resins include methoxylated methylol urea, methoxylated methylol N, N-ethylene urea, methoxylated methylol dicyandiamide, methoxylated methylol melamine, methoxylated methylol benzoguanamine, butoxylated methylol melamine, butoxylated methylol benzoguanamine, and the like. Are preferably methoxylated methylol melamine, butoxylated methylol melamine, methylolated benzoguanamine, and the like.

  Examples of the polyfunctional epoxy compound include diglycidyl ether of bisphenol A and oligomer thereof, diglycidyl ether of hydrogenated bisphenol A and oligomer thereof, orthophthalic acid diglycidyl ester, isophthalic acid diglycidyl ester, terephthalic acid diglycidyl ester, p-oxybenzoic acid diglycidyl ester, tetrahydrophthalic acid diglycidyl ester, hexahydrophthalic acid diglycidyl ester, succinic acid diglycidyl ester, adipic acid diglycidyl ester, sebacic acid diglycidyl ester, ethylene glycol diglycidyl ether, propylene Glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether and Polyalkylene glycol diglycidyl ethers, trimellitic acid triglycidyl ester, triglycidyl isocyanurate, 1,4-diglycidyloxybenzene, diglycidyl propylene urea, glycerol triglycidyl ether, trimethylolpropane triglycidyl ether, pentaerythritol triglycidyl Examples include ether, triglycidyl ether of glycerol alkylene oxide adduct, and the like.

  As the polyfunctional isocyanate compound, a low-molecular or high-molecular aromatic or aliphatic diisocyanate or a trivalent or higher polyisocyanate can be used. Examples of the polyisocyanate include tetramethylene diisocyanate, hexamethylene diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, isophorone diisocyanate, and trimers of these isocyanate compounds. Furthermore, an excess amount of these isocyanate compounds and low molecular active hydrogen compounds such as ethylene glycol, propylene glycol, trimethylolpropane, glycerin, sorbitol, ethylenediamine, monoethanolamine, diethanolamine, triethanolamine, or polyester polyols, poly The terminal isocyanate group containing compound obtained by making it react with polymer active hydrogen compounds, such as ether polyols and polyamides, can be mentioned.

  The blocked isocyanate can be prepared by subjecting the above isocyanate compound and blocking agent to an addition reaction by a conventionally known appropriate method. Examples of the isocyanate blocking agent include phenols such as phenol, cresol, xylenol, resorcinol, nitrophenol and chlorophenol; thiophenols such as thiophenol and methylthiophenol; oximes such as acetoxime, methyl etiketooxime and cyclohexanone oxime. Alcohols such as methanol, ethanol, propanol and butanol; halogen-substituted alcohols such as ethylene chlorohydrin and 1,3-dichloro-2-propanol; tertiary alcohols such as t-butanol and t-pentanol ; Lactams such as ε-caprolactam, δ-valerolactam, ν-butyrolactam, β-propyllactam; aromatic amines; imides; acetylacetone, acetoacetate, ethyl malonate Active methylene compounds such as esters, mercaptans, imines; ureas; diaryl compounds; sodium bisulfite, and the like.

(Iv) Other additives In the antistatic layer, a surfactant used for improving leveling properties, an inorganic material that improves handling properties such as slipping properties and blocking properties, as long as the effects of the present invention are not impaired. Additives such as organic particles, waxes that control slipperiness and surface energy, and the like can be included. Moreover, you may mix | blend polyolefin resin or a fluorine resin for the improvement of blocking property. These additives are contained in the coating solution.

(V) Method for forming antistatic layer In the present invention, the antistatic agent layer is formed by applying and drying a coating liquid containing a binder resin, an antistatic agent, a solvent, and other additives as necessary on the surface of the plastic film. It is formed by heat treatment as necessary. In particular, the plastic film has a laminated structure with B / A or B / A / B as a unit, and the B layer is a polyester layer containing a polyester resin and particles, and does not substantially contain a cavity. When the layer is a void-containing laminated polyester film that is a void-containing polyester layer composed of a composition in which a polyester resin and a thermoplastic resin incompatible with the polyester resin are mixed, the antistatic layer is a laminate of the rewrite printing layer of the laminated film. It is preferable to form it on the surface opposite to the B layer on the side.

  The coating solution is preferably an aqueous solution or an aqueous dispersion because an organic solvent that causes a problem with the environment is not used. The solid content concentration in the coating solution is usually 1% by mass to 50% by mass, preferably 3% by mass to 30% by mass.

  Examples of the method for applying this coating solution to a plastic film include reverse roll coating, gravure coating, kiss coating, roll brushing, spray coating, air knife coating, wire barber coating, and pipe doctor method. , Impregnation / coating method, curtain coating method and the like, and these methods can be carried out alone or in combination.

  In the present invention, the formation of the antistatic layer on the plastic film is preferably carried out by a so-called in-line coating method which is carried out in the plastic film production process. That is, it may be carried out by a method in which the above-mentioned coating solution is applied to an unstretched or uniaxially stretched plastic film, dried, further uniaxially stretched or biaxially stretched, and then heat-set.

As the binder resin of the antistatic layer, using the above polyester-based graft polymer,
When the coating layer is formed by the in-line coating method, the drying temperature is preferably 70 to 140 ° C., and the drying time is adjusted according to the solid content concentration and the coating amount of the coating solution, but the temperature (° C.) and time ( (Second) product is preferably 3,000 or less. When the product exceeds 3,000, the coating layer undergoes a crosslinking reaction before stretching, and cracks are likely to occur in the coating layer.

  Moreover, after extending | stretching a coating layer, in the state which fixed the film so that film width length may not change, it is bridge | crosslinking by heat-processing a coating layer with an infrared heater at 250-260 degreeC for 0.5 to 1 second for a short time. It is preferable for promoting the reaction.

  Under the present circumstances, when 1-10 mass% of acid compounds are added with respect to resin in a coating liquid, a crosslinking reaction is further accelerated | stimulated and an application layer will become firmer. Therefore, the oligomer which precipitates on the film surface when the film is heated can be blocked by the coating layer, and oligomer deposition on the surface of the coating layer can be suppressed.

  The film after stretching is usually subjected to a relaxation treatment of about 2 to 10%, but in the present invention, the coating layer is coated with an infrared heater in a state where the coating layer is less distorted, i.e., fixed so that the film width does not change. It is preferable to heat. By adopting such a method, crosslinking in the coating layer is promoted and becomes stronger. If the heating temperature or time is greater than the above conditions, crystallization or dissolution of the film tends to occur. On the other hand, if the one condition is a heating temperature or time smaller than the above condition, crosslinking of the coating layer tends to be insufficient.

The coating amount after drying of the finally obtained coating layer (solid content mass per unit film area) is preferably 0.01 to 0.50 g / m ² , more preferably 0.02 to 0.40 g / m 2. ² , particularly preferably 0.05 to 0.30 g / m ² . When the coating amount after drying is less than 0.01 g / m ² , adhesion and durability tend to be insufficient. On the other hand, when the coating amount exceeds 0.50 g / m ² , it tends to be easily blocked, and the number of foreign matters in the coating layer is also likely to increase.

In addition, it is attached to the film surface that the cleanness in the air in the coating process (number of particles of 0.5 μm or more / ft ³ ) is controlled by a hepa filter so as to be class 100,000 after the unstretched film is created. It is effective in reducing foreign matter.

(3) Adhesion-improving layer In the base film for a rewritable recording medium of the present invention, an adhesion-improving layer may be provided on the surface opposite to the antistatic layer, that is, the surface on which the rewritable printing layer is laminated. This is a preferred embodiment. Further, the plastic film has a laminated structure with B / A or B / A / B as a unit, and the B layer does not substantially contain a cavity therein, and more preferably, the three-dimensional center plane average surface roughness. The most preferable embodiment is to use a void-containing laminated polyester film in which (SRa) is 0.07 to 0.30 μm and the A layer is composed of a void-containing polyester layer, and to provide an adhesion improving layer on the surface of the B layer. It is.

  As the resin constituting the adhesion improving layer, the resin exemplified for the binder resin used for the antistatic layer can be used. Examples thereof include polyester, polyurethane, acrylic resin, vinyl resin, epoxy resin, amide resin, polyvinyl alcohol, or a copolymer of each resin. These resins may be resins grafted by introducing a copolymer component into the main chain of each resin. Examples of the graft resin include acrylic resin graft polyester, acrylic resin graft polyurethane, vinyl resin graft polyester, and vinyl resin graft polyurethane.

  Among these resins, a polyester graft polymer obtained by graft polymerization of a polymerizable unsaturated monomer containing at least one carboxyl group residue to a polyester resin containing an ethylene glycol residue is used to form an adhesion improving layer. When used as a resin, it is possible to improve the adhesion to the rewritable printing layer, particularly the wet heat resistance.

  As a result, the adhesion between the rewritable recording medium substrate film of the present invention and the rewritable printing layer is improved, and it has excellent durability when repeated coloring and decoloring. The rewrite printing layer is highly durable.

  Furthermore, by forming a coating layer containing the above graft polymer as a constituent component on both sides of the plastic film (antistatic layer and adhesion improving layer), the cyclic oligomer and unreacted monomer contained in the plastic film are formed. It can suppress that a low molecular weight thing precipitates on the film surface.

When a coating layer containing the above polyester-based graft polymer as a constituent component is provided on the film surface, absorption maxima are observed at two wavelengths of 1781 ± 10 cm ⁻¹ and 1341 ± 10 cm ⁻¹ . Here, the absorption having an absorption maximum at 1781 ± 10 cm ⁻¹ is attributed to an acid anhydride residue, and the absorption having an absorption maximum at 1341 ± 10 cm ⁻¹ is attributed to an ethylene glycol residue. In the present invention, when the absorbance of absorption having an absorption maximum at a wavelength of 1781 ± 10 cm ⁻¹ is B and the absorbance of absorption having an absorption maximum at 1341 ± 10 cm ⁻¹ is A, the absorbance ratio T1 (= B / A) is preferably 0.18 or more. That is, the characteristics reflect the concentration of acid anhydride residues on the surfaces of the antistatic layer and the adhesion improving layer.

  In the present invention, the coating solution containing the graft polymer constituting the antistatic layer or the adhesion improving layer is preferably used as an aqueous solution or an aqueous dispersion from the viewpoint of environmental load. In order to impart hydrophilicity to the graft polymer when the aqueous solution or aqueous dispersion is used, the acid anhydride bond in the graft polymer is converted into a carboxyl group or a salt thereof by hydrolysis. Therefore, the concentration of the acid anhydride residue on the surface of the coating layer reflects the degree of self-crosslinking of the graft polymer after forming the above-mentioned adhesion improving layer. That is, the higher the concentration of the acid anhydride residue (the higher the absorbance ratio T1), the higher the degree of self-crosslinking of the graft polymer.

  The absorbance ratio T1 (= B / A) is more preferably 0.20 or more, and further preferably 0.22 or more. If the absorbance ratio T1 (= B / A) is less than 0.18, the adhesion of the rewritable printing layer is lowered, which is not preferable. The upper limit is not limited, but if it is 0.32 or more, the effect of adhesion of the rewrite print layer is saturated.

  The reason why low molecular weight components such as polyester oligomers contained in the polyester resin can be prevented from shifting to the surface of the antistatic layer by satisfying the above characteristics is not clear, but by increasing the acid anhydride residue It is presumed that the effect of decreasing the polarity of the coating layer contributes. In addition, it is considered that an increase in acid anhydride residues includes a reaction product due to an intermolecular reaction. Therefore, it is presumed that the cross-linking reaction of the graft polymer constituting the coating layer proceeds and the packing effect is increased.

  In the present invention, as a method for controlling the infrared absorption characteristics of the surface of the coating layer within the above range, for example, a base film provided with the coating layer using the graft polymer as a resin for forming the coating layer A method of heat-treating through a heating zone of 200 to 250 ° C. is preferable.

(4) Rewrite recording medium The rewrite recording medium includes a base film and a rewrite printing layer laminated on the side opposite to the antistatic layer of the base film. Further, in practice, the rewritable recording medium is often used in a configuration in which a protective layer is provided on the surface of the rewritable print layer and an adhesive layer is provided on the uppermost layer.

  As the leuco dye, the reversible developer, and the binder resin constituting the rewritable printing layer of the rewritable recording medium, known commercially available products can be used. Further, as a means for recording / erasing the rewritable recording medium in a non-contact manner, a laser having a specific wavelength such as a semiconductor laser, a carbon dioxide gas laser, or a YAG laser, a lamp having a width in wavelength, or the like can be used.

  Hereinafter, the present invention will be described in detail by way of examples. The characteristics of the film-writable recording medium obtained in each example were measured and evaluated by the following methods.

(1) Intrinsic viscosity 0.1 g of the chip sample was precisely weighed and dissolved in 25 ml of a mixed solvent of phenol / tetrachloroethane = 6/4 (mass ratio), and measured at 30 ° C. using an Ostwald viscometer. In addition, the measurement was performed 3 times and the average value was calculated | required.

(2) Three-dimensional center plane average surface roughness (SRa)
Using a stylus type three-dimensional surface roughness meter (SE-3AK, manufactured by Kosaka Laboratory Ltd.), the surface of the film was subjected to the conditions of a needle radius of 2 μm, a load of 30 mg, and a needle speed of 0.1 mm / sec. In the longitudinal direction of the film, the cut-off value is 0.25 mm, the measurement length is 1 mm, and it is divided into 500 points at a 2 μm pitch, and the height of each point is determined by a three-dimensional roughness analyzer (Kosaka Laboratory Co., Ltd.) Manufactured by TDA-21). The same operation was performed 150 times continuously at intervals of 2 μm in the width direction of the film, that is, over 0.3 mm in the width direction of the film, and the data was taken into the analyzer. Next, the three-dimensional average surface roughness SRa was determined using the analyzer. The unit of SRa is μm. In addition, the measurement was performed 3 times and those average values were employ | adopted.

(3) Film thickness Using a Millitron thickness meter, a total of 15 points were measured, with 5 points per film, and the average value was determined.

(4) Layer thickness of film The film was cut using a microtome to obtain a cross section perpendicular to the film surface. A sample obtained by coating the cross section with a platinum / palladium alloy by sputtering was used as an observation sample. The cross section of the film was observed using a scanning electron microscope (manufactured by Hitachi, Ltd., model S-510), and a photograph was taken at an appropriate magnification at which the total thickness of the film became one field of view. From this image, the thickness of each layer was measured using a scale. Measurement was performed on three independently prepared cross-sectional samples, and the average value was taken as the layer thickness of the laminated film.

(5) Light transmittance of film In accordance with JIS-B0601-1982, Poic integrating sphere type H.264 is used. T.A. Measurement was performed using an R meter (manufactured by Nippon Seimitsu Optics). The smaller this value is, the higher the light shielding property.

(6) Apparent density of film Four films were cut into 5.0 cm squares and used as samples. Four samples were stacked, and the total thickness was measured at 10 points using a micrometer with four significant figures, and the total value of the stacked thickness was determined. The average value was divided by 4 and rounded to 3 significant figures to obtain an average thickness per sheet (t: μm). The mass (w: g) of the four samples was measured using an automatic upper pan balance with four significant digits, and the apparent density was determined from the following equation. The apparent density was rounded to 3 significant figures.
Apparent density (g / cm ³ ) = (w / (5.0 × 5.0 × 4 × t)) × 10 ⁴

(7) Thermal conductivity Measured by a steady comparison method according to ASTM E1530. UNITHERM2010 (manufactured by ANTER) was used as a measuring instrument. As a sample, a film laminated with a total thickness of about 1 mm was used, and the thermal conductivity in the thickness direction was measured at 120 ° C.

(8) Surface Cleavage Resistance of Film A cross cut of 100 grids is made at 1 mm intervals on the film surface on the side where the rewritable printing layer of the film sample is laminated, and 24 mm wide × 40 mm centering on the cross cut portion. Adhesive tape (manufactured by Nichiban Co., Ltd., CT405AP-24) is attached so as to have a long area. Next, after applying a load of 1 kgf / cm ^{2 to} the affixed portion for 10 seconds, when the adhesive tape was peeled off in the vertical direction of the film, the number peeled off to the adhesive tape side was counted, and the following criteria were used. Judged.
○: 0 to 20 pieces / 100 pieces △: 21 to 50 pieces / 100 pieces ×: 51 to 100 pieces / 100 pieces

(9) Surface resistivity value The film was allowed to stand in an atmosphere of 23 ° C. and 65% RH for 24 hours, and then the surface resistance value measuring device (manufactured by Mitsubishi Yuka Co., Ltd., Hiresta IP) was used to apply an applied voltage of 500V. The surface resistivity value (Ω / □) of the surface of the antistatic agent layer was measured.

(10) Infrared absorption analysis method The film to be measured is sampled to about 2 cm × 2 cm, the measurement target surface is brought into close contact with the ATR crystal and set in an IR measuring apparatus, and the IR spectrum is measured by the following method. The baseline was a line connecting the hems on both sides of each maximum absorption peak.
Apparatus: FT-IR FTS-60A / 896 (manufactured by Varian Technologies Japan Limited)
Single reflection ATR attachment: silver gate (manufactured by SPECAC)
ATR crystal: Ge
Incident angle: 45 °
Resolution: 4cm ^-1
Integration count: 128 times

The absorbance ratio T1 (= B / A) of the absorbance B having an absorption maximum at 1781 ± 10 cm ⁻¹ and the absorbance A having an absorption maximum at 1341 ± 10 cm ⁻¹ was determined. Here, the absorption having an absorption maximum at 1781 ± 10 cm ⁻¹ is attributed to an acid anhydride residue, and the absorption having an absorption maximum at 1341 ± 10 cm ⁻¹ is attributed to an ethylene glycol residue.

(11) Occupied area ratio of surface-deposited particles by heat treatment (a) Heat treatment A small piece is cut out from any 5 points of the film to be measured, the end is gripped with a snake-eye clip and heated in hot air at 170 ° C. for 20 minutes. did. At this time, the film was held so as not to come into contact with other films and instruments, and handled so as not to cause scratches. After heating, it was taken out to room temperature and sufficiently cooled naturally, and then the following observation was performed.

(B) Ratio of Occupied Area of Precipitated Particles on Film Surface First, dust and the like were carefully removed from a film piece to be measured by a static elimination blower. This surface was measured with a non-contact type three-dimensional shape measuring device (manufactured by Micromap; Micromap 557). The optical system used was a mirro-type two-beam interference objective lens (50 ×) and a zoom lens (Body Tube, 0.5 ×), and received light with a 2/3 inch CCD camera using a light source of 5600 Å. The measurement was performed in the WAVE mode, and a field of view of 245 μm square was processed as a digital image of 480 pixels. For analysis of the image, surface shape data was obtained by using an analysis software (Micromap 123, version 4.0) and performing detrending in the quartic function mode. Particle analysis was performed from the shape data using analysis software (SX-Viewer, version 3.4.2). After performing planar correction and interpolation using the correction function of the software, protrusions having a longest diameter of 0.01 to 2000 μm and a height of 0.1 μm to 1000 μm were analyzed. As projection analysis parameters, a binarization threshold value 0.01, a rebinarization threshold value 50, and a block size 4 were given, and the projections were binarized and extracted. The area occupied by the protrusions was determined from the obtained analysis results, and the ratio of the area occupied by the protrusions and the area of the visual field (6.0 × 10 ⁴ μm ² ) increased before and after the heat treatment was defined as the occupied area ratio. The measurement was performed on five pieces of film, each at any three locations avoiding clear scratches and foreign matters, and the average value in a total of 15 fields was obtained and used.

(12) Color development property of rewritable printing layer The rewritable recording medium was colored with an applied energy of 0.7 mJ / dot using a commercially available thermal card printer. After color development, it was rapidly cooled to room temperature, and the color developability was judged. The color development was determined by classifying 1 to 5 grades by visual observation. Grade 1 was the best and Grade 5 was the worst.

(13) Decoloring property of rewrite printing layer Printing was performed by the method described above, and then decoloring was performed with an applied energy of 0.56 mJ / dot. The decoloring property was determined by classifying it into 1 to 5 grades by visual observation. Grade 1 was the best and Grade 5 was the worst.

(14) Fineness of rewrite print image The dot shape of the print image of the print sample was determined by observing with a microscope. The closer the dot shape is to a square, the better the definition. Judgment was made according to the following criteria.
◎: Roughly square ○: Slightly rounded △: Slightly distorted due to white spots etc. ×: Shrimp

(15) Sharpness of rewrite print image The print image developed by the above method was determined by visual observation.
◯: Good sharpness ×: Poor sharpness

Example 1
[Preparation of Cavity Forming Agent-Containing Master Pellet (I)]
As one of the raw materials for the intermediate layer, 20% by mass of a polystyrene resin having a melt flow rate of 1.5 (manufactured by Nippon Polyste, G797N), a gas phase polymerization polypropylene resin having a melt flow rate of 3.0 (manufactured by Idemitsu Petrochemical Co., Ltd., F300SP) 20% by mass and a melt flow rate 180 polymethylpentene resin (Mitsui Chemicals Co., Ltd .: TPX DX-820) 60% by mass are mixed with pellets, supplied to a twin-screw extruder and kneaded sufficiently, The cavity forming agent-containing master pellet (I) was prepared by cooling and cutting.

[Preparation of titanium oxide-containing master pellets (b)]
As one of the raw materials for the intermediate layer, anatase-type titanium dioxide (TA-300, manufactured by Fuji Titanium Co., Ltd.) having an average viscosity of 49.5% by mass of polyethylene terephthalate resin with an intrinsic viscosity of 0.62 dl / g and an average particle size of 0.3 μm (electron microscopy). ) A mixture of 50% by mass and 0.05% by mass of optical brightener (manufactured by Eastman Chemical Co., Ltd., OB-1) was supplied to a vented twin screw extruder and pre-kneaded, and then the molten polymer was continuously used. Then, the mixture was supplied to a vent type single-screw kneader and kneaded to prepare titanium oxide-containing master pellets (b).

[Preparation of coating solution for forming antistatic layer]
(Preparation of copolyester resin)
In a stainless steel autoclave equipped with a stirrer, a thermometer, and a partial reflux condenser, 345 parts by weight of dimethyl terephthalate, 211 parts by weight of 1,4-butanediol, 270 parts by weight of ethylene glycol, and tetra-n-butyl titanate 0.5 part by mass was charged, and a transesterification reaction was performed from 160 ° C. to 220 ° C. over 4 hours. Next, 14 parts by mass of fumaric acid and 160 parts by mass of sebacic acid were added, and the temperature was raised from 200 ° C. to 220 ° C. over 1 hour to carry out an esterification reaction. Next, the temperature was raised to 255 ° C., the pressure of the reaction system was gradually reduced, and the mixture was reacted for 1 hour 30 minutes under a reduced pressure of 29.3 Pa to obtain a copolyester. The obtained copolyester was light yellow and transparent and had a weight average molecular weight of 20,000. Moreover, the ratio of the aromatic component by NMR analysis was 70 mol%.

(Preparation of water-dispersible graft resin)
A reactor equipped with a stirrer, a thermometer, a reflux device and a quantitative dropping device was charged with 75 parts by mass of copolyester, 56 parts by mass of methyl ethyl ketone and 19 parts by mass of isopropyl alcohol, and heated and stirred at 65 ° C. to dissolve the resin. . After the resin was completely dissolved, 15 parts by weight of maleic anhydride was added to the polyester solution.
Next, a solution prepared by dissolving 10 parts by mass of styrene and 1.5 parts by mass of azobisdimethylvaleronitrile in 12 parts by mass of methyl ethyl ketone was dropped into the polyester solution at 0.1 ml / min, and stirring was further continued for 2 hours. After sampling for analysis from the reaction solution, 5 parts by mass of methanol was added. Next, 300 parts by mass of water and 15 parts by mass of triethylamine were added to the reaction solution and stirred for 1 hour. Thereafter, the reactor internal temperature was raised to 100 ° C., and methyl ethyl ketone, isopropyl alcohol and excess triethylamine were distilled off to obtain a water-dispersible graft resin. This water-dispersible graft resin was light yellow and transparent. The acid value of this resin was 1,400 eq / t.

(Preparation of coating solution)
40 parts by weight of a 25% by weight aqueous dispersion of the water-dispersible graft resin, 4.3 parts by weight of polystyrene sulfonate ammonium salt (manufactured by NSC Japan, molecular weight 70,000), 24 parts by weight of water and isopropyl alcohol 36 parts by mass of each, and further colloidal silica particles (Nissan Chemical Co., Ltd.) dispersed by a homogenizer in ion-exchanged water and 0.6 parts by mass of a 10% by mass aqueous solution of an anionic surfactant and 1 part by mass of propionic acid. Dry silica particles (manufactured by Nippon Aerosil Co., Ltd.) obtained by dispersing a 20 mass% aqueous dispersion of Snowtex OL, average particle size 40 nm (particle a), manufactured by Kogyo Co., Ltd., with 1.8 parts by mass of homogenizer in ion-exchanged water. (Aerosil OX50, average aggregate particle size 200 nm, average primary particle size 40 nm) (particle b) Parts was added, and the coating solution. The mass ratio of the particles a and the particles b is 8, and the content of the particles b is 0.42% by mass with respect to the resin composition of the coating layer.

[Manufacture of base film for rewrite recording medium]
A mixture of 7% by mass of the above-mentioned cavity forming agent-containing master pellets (A), 7% by mass of titanium oxide-containing master pellets (B) and 86% by mass of PET resin having an intrinsic viscosity of 0.62 dl / g was used as a raw material for the intermediate layer. Also, a mixture of 30% by mass of titanium oxide-containing master pellets (ii) and 70% by mass of PET resin having an intrinsic viscosity of 0.62 dl / g is supplied to two extruders so as to be laminated on both sides of the intermediate layer. These were joined with a feed block so that the thickness ratio of surface layer / intermediate layer / surface layer was 8/84/8. Subsequently, it extruded on the cooling roll adjusted to 20 degreeC from the die | dye, and manufactured the unstretched film of the 3 layer structure of thickness 1.6mm. At the time of extrusion, the polyester resin and the mixture of the polyester resin and the master pellet were previously dried in vacuum and then supplied to the extruder. The cooling drum was cooled by blowing cold air adjusted to 20 ° C. on the opposite surface of the cooling drum. In addition, as said PET resin, all used the general purpose resin in which the low oligomerization process is not carried out. The content of the cyclic trimer in the polyester resin was 9,600 ppm.

  The obtained unstretched film is uniformly heated to 65 ° C. using a heating roll, and 3.4 between two pairs of nip rolls (low speed roll: 1 m / min, high speed roll: 3.4 m / min) having different peripheral speeds. The uniaxially stretched PET film containing voids was obtained by stretching it twice. At this time, as an auxiliary heating device for the film, an infrared heater (rated: 40 W / cm) provided with a gold reflective film in the middle part of the nip roll was placed opposite to both sides of the film (a distance of 1 cm from the film surface). Was heated at 18 W / cm and the opposite surface was heated at 12 W / cm.

The antistatic layer-forming coating solution is microfiltered with a felt-type polypropylene filter medium having a particle size of 10 μm (initial filtration efficiency of 95%), and is applied to one side of the cavity-containing uniaxially stretched PET film by a reverse roll method. And then dried. The coating amount of the antistatic layer after drying was 0.1 g / m ² .

  Subsequently, the end of the film was gripped with a clip, guided to a hot air zone heated to 130 ° C. and dried, and then stretched 4.0 times in the width direction. Further, a 188 μm-thick void-containing biaxially stretched polyester film having an antistatic layer on one side was manufactured by heating with an infrared heater at 250 ° C. for 0.6 seconds with the width of the film fixed, and rewritable. A base film for a recording medium was obtained. Table 1 shows the characteristic values of the obtained base film for a rewritable recording medium. In addition, the whiteness of the obtained base film for rewritable recording media was 84. Further, the surface of the antistatic layer of the obtained base film for rewritable recording medium has a large absorbance ratio T1 as evaluated by infrared absorption analysis of 0.28, a high acid anhydride concentration, and a degree of self-crosslinking. Is high.

[Manufacture of rewrite recording media]
20 parts 3-di-n-butylamino-6-methyl-7-anilinofluorane as leuco dye and N- [3- (p-hydroxyphenyl) propiono] -N'-n- as reversible developer For rewrite printing layer, 100 parts of octadecanohydrazide and 80 parts of polyvinyl butyral (BX-1) as binder resin are ground and dissolved in a ball mill for 24 hours to form a 10% by mass methyl ethyl ketone solution. A coating solution was prepared.

The rewritable coating layer coating solution is applied to the surface opposite to the antistatic layer of the rewritable recording medium base film so that the thickness is 3 μm after drying by the wire bar method and dried at 80 ° C. for 2 minutes. Thus, a rewritable recording medium having a rewritable printing layer was obtained.
The rewrite printing characteristics of the obtained rewrite recording medium were evaluated. The results are shown in Table 1.

  The base film for rewritable recording medium obtained in Example 1 and the rewritable recording medium obtained therefrom were excellent in all characteristics and high quality.

Example 2
In Example 1, the rewrite of Example 2 was performed in the same manner as in Example 1 except that the compounding amounts of the void forming agent-containing master pellets (A) and the PET resin in the intermediate layer were changed to 9 and 84% by mass, respectively. A base film for recording medium and a rewritable recording medium were obtained. Table 1 shows the properties of the obtained base film for a rewrite recording medium and the rewrite recording medium.

  The base film for rewritable recording medium and the rewritable recording medium obtained in Example 2 have the same characteristics as the base film for rewritable recording medium and the rewritable recording medium obtained in Example 1, and have high quality. Met.

Example 3
In Example 1, as a polyester resin composition for forming an intermediate layer, 7% by mass of titanium oxide-containing master pellets (b) and hollow glass beads having an average particle diameter of 5 μm (prepared by classifying commercially available glass beads) 5% by mass The base film for rewritable recording medium and the rewritable recording medium of Example 3 were obtained in the same manner as in Example 1 except that the mixture was changed to a kneaded product of 88% by mass of PET resin having an intrinsic viscosity of 0.62 dl / g. Obtained. Table 1 shows the properties of the obtained base film for a rewrite recording medium and the rewrite recording medium.

  The base film for rewritable recording medium and the rewritable recording medium obtained in Example 3 have the same characteristics as the base film for rewritable recording medium and the rewritable recording medium obtained in Example 1, and have high quality. Met.

Comparative Example 1
In the method of Example 1, the base material for the rewritable recording medium of Comparative Example 1 was used in the same manner as in Example 1 except that the compounding of the cavity forming agent-containing master pellets (A) into the polyester resin composition for forming the intermediate layer was stopped. Films and rewritable recording media were obtained. Table 1 shows the properties of the obtained base film for a rewrite recording medium and the rewrite recording medium.

  The rewritable recording medium substrate film obtained in Comparative Example 1 is inferior in heat insulation to the rewritable recording medium substrate film obtained in Example 1. Therefore, the erasability was inferior to the rewritable recording medium obtained in Example 1.

Comparative Example 2
In the method of Example 1, for the rewritable recording medium of Comparative Example 2 in the same manner as in Example 1 except that the compounding of the titanium oxide masterbatch (B) into the polyester resin composition is stopped as the raw material used in the intermediate layer. A base film and a rewritable recording medium were obtained. Table 1 shows the properties of the obtained base film for a rewrite recording medium and the rewrite recording medium.

  The base film for rewritable recording medium obtained in Comparative Example 2 is inferior in light-shielding property to the base film for rewritable recording medium obtained in Example 1, and the printed printed image is sharp. It was inferior.

Comparative Example 3
One side of a commercially available white PET film is 13% by mass of 92% hollow particles made of a copolymer resin mainly composed of vinylidene chloride-acrylonitrile having an average particle size of 5 μm, and 3% by mass of styrene / butadiene copolymer latex. A base film for a rewritable recording medium was obtained by applying a heat-insulating coat coating solution comprising a dispersion composed of (solid content equivalent amount) and 84% by mass of water to a thickness of 10 μm after drying and drying. In addition, the calendar process of this film was not implemented. Moreover, the printing layer was laminated | stacked on the said heat insulation coating layer side.
Table 1 shows the characteristics of the obtained rewritable recording medium and the rewritable recording medium obtained by laminating the rewritable printing layer in the same manner as in Example 1.

  The base film for rewritable recording medium obtained in Comparative Example 3 is inferior in heat insulation to the base film for rewritable recording medium obtained in Example 1. Therefore, the erasability was inferior to the rewritable recording medium obtained in Example 1. Moreover, the surface roughness of the heat insulation coating layer surface of the base film for rewritable recording media obtained in this Comparative Example was rough, and the fineness of the printed image was inferior. Furthermore, the antistatic property was inferior, and the suppression of precipitation of low molecular weight components in the polyester on the film surface was insufficient. Further, the surface cleavage resistance of the film surface on the side where the rewrite printing layer was laminated was slightly inferior.

Comparative Example 4
In Example 1, Comparative Example 4 was carried out in the same manner as in Example 1, except that the blending amounts of the void forming agent-containing master pellets (A) and the PET resin in the intermediate layer were changed to 15% by mass and 78% by mass, respectively. A base film for a rewrite recording medium and a rewrite recording medium were obtained. Table 1 shows the properties of the obtained base film for a rewrite recording medium and the rewrite recording medium.

  The rewritable recording medium base film obtained in Comparative Example 4 has a lower apparent density and a higher cavity content than the rewritable recording medium base film obtained in Example 1. Therefore, since the heat insulating property was too good, the color development was inferior to the rewritable recording medium obtained in Example 1. Further, the surface cleavage resistance of the film surface on the side where the rewrite printing layer was laminated was slightly inferior.

Comparative Example 5
In the method of Comparative Example 2, when forming the void-containing polyester film, the same method as in Example 1 except that only the intermediate layer A is formed as a single layer without laminating the surface layer B that does not substantially contain voids. By the method, the base film for rewrite recording medium and the rewrite recording medium of Comparative Example 5 were obtained. Table 1 shows the properties of the obtained base film for a rewrite recording medium and the rewrite recording medium.

  The base film for rewritable recording medium obtained in Comparative Example 5 is not laminated with a surface forming layer in addition to the problems of the base film for rewritable recording medium obtained in Comparative Example 2. Furthermore, the surface cleavage resistance of the film surface on the side where the rewritable printing layer is laminated was inferior.

Comparative Example 6
In the method of Example 1, a base film for rewrite recording medium and a rewrite recording medium of Comparative Example 6 were obtained in the same manner as in Example 1 except that the antistatic layer was not formed in the film forming step. Table 1 shows the properties of the obtained base film for a rewrite recording medium and the rewrite recording medium.
The base film for a rewritable recording medium obtained in Comparative Example 6 was inferior in antistatic properties, and the precipitation of low molecular weight components in the polyester on the film surface was insufficient.

Comparative Example 7
In the method of Comparative Example 2, a void-containing laminated polyester film was obtained in the same manner as in Comparative Example 2 without forming an antistatic layer during film formation. Next, after producing the void-containing laminated polyester film, Comparative Example 7 was prepared in the same manner as Comparative Example 2 except that the antistatic layer described in Comparative Example 2 was formed and the coating layer was formed by the so-called offline coating method. A base film for a rewrite recording medium and a rewrite recording medium were obtained. Table 1 shows the properties of the obtained base film for a rewrite recording medium and the rewrite recording medium.

  The base film for rewritable recording medium obtained in Comparative Example 7 has a low molecular weight component antistatic layer in polyester in addition to the deterioration in quality of the base film for rewritable recording medium obtained in Comparative Example 2. The effect of suppressing precipitation on the surface was insufficient, and the occupied area ratio of particles deposited on the surface of the antistatic layer was high. When the surface of the rewritable layer adhesion improving layer and the antistatic layer of the rewritable recording medium substrate film obtained in Comparative Example 7 was evaluated by infrared absorption analysis, the absorbance ratio T1 was as small as 0.03. The acid anhydride concentration was low. That is, the degree of self-crosslinking was insufficient.

Comparative Example 8
Comparative Example 2 except that the water-dispersible graft resin used in the coating solution for the antistatic layer is changed to a water-dispersible copolyester resin (manufactured by Toyobo Co., Ltd., Bironal MD-16) in the method of Comparative Example 2. The base film for rewritable recording medium of Comparative Example 8 and the rewritable recording medium were obtained in the same manner as in No. 2. Table 1 shows the properties of the obtained base film for a rewrite recording medium and the rewrite recording medium.
The base film for rewritable recording medium obtained in Comparative Example 8 was charged with a low molecular weight component contained in the polyester film in addition to the deterioration in quality of the base film for rewritable recording medium obtained in Comparative Example 2. The effect of suppressing the precipitation on the surface of the prevention layer was insufficient, and the occupied area ratio of the particles deposited on the surface of the antistatic layer was high.

  FIG. 1 shows the relationship between the thermal conductivity of the base film for a rewrite recording medium and the color developability and decolorability of the rewrite recording medium using the data of Examples and Comparative Examples. From FIG. 1, it can be understood that the limited range of the present invention is a critical range.

  The substrate film for rewritable recording medium of the present invention, when used as a substrate for a rewritable recording medium, is excellent in the balance between color development and decoloring by heat energy, and is excellent in the visibility of the printed color, Since it is excellent in antistatic properties, it is suitable as a base film for rewritable recording media and greatly contributes to the industry.

It is explanatory drawing which shows the relationship between the heat conductivity of the base film for rewrite recording media, and the coloring property and decoloring property of a rewrite recording medium using the data of an Example and a comparative example.

Claims (7)

A rewritable recording medium substrate film for use in a rewritable recording medium in which a rewritable printing layer is laminated on one side of the substrate film, wherein the rewritable recording medium substrate film has a side on which a rewritable printing layer of a plastic film is laminated. Has an antistatic layer comprising an antistatic agent and a binder resin on the opposite surface, has a light transmittance of 15% or less, and a thermal conductivity of 0.037 to 0.080 W / mK. A base film for a rewritable recording medium, wherein the surface resistivity (25 ° C., 65% RH) value of the antistatic layer is 1 × 10 ¹³ Ω / □ or less. The base film for a rewritable recording medium according to claim 1, wherein the plastic film is a void-containing polyester film having an apparent density of 0.91 to 1.2 g / cm ³ .   The void-containing polyester film has a laminated structure in units of B / A or B / A / B. The B layer is a polyester layer containing a polyester resin and particles, and a rewritable printing layer is laminated when used as a rewritable recording medium. 3. The rewritable recording medium according to claim 2, wherein the layer A is a void-containing polyester layer comprising a composition in which a polyester resin and a thermoplastic resin incompatible with the polyester resin are mixed. Base film.   The rewrite recording according to claim 3, wherein the B layer is a layer that does not substantially contain a cavity therein, and has a three-dimensional center plane average surface roughness (SRa) of 0.07 to 0.30 μm. Base film for media.   The antistatic agent contained in the antistatic layer is (a) an anionic polymer compound having at least one sulfonate group or phosphate group in the molecule, and (b) a cationic polymer compound containing a quaternary ammonium base. The base film for a rewritable recording medium according to any one of claims 1 to 4, wherein the base film is selected from (c) polythiophene or a derivative thereof.   The binder resin constituting the antistatic layer is a polyester graft polymer obtained by graft polymerization of a polymerizable unsaturated monomer containing at least one carboxyl group residue on a polyester resin containing an ethylene glycol residue. The base film for rewritable recording media according to any one of claims 1 to 5.   A rewritable recording medium comprising a rewritable printing layer laminated on the surface opposite to the antistatic layer of the substrate film for a rewritable recording medium according to claim 1.

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