JP2002137541A - Double-walled film microcapsule and image-recording sheet using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a double-walled film microcapsule suitably usable for pressure and heat sensitive recording wherein a wax outer wall which has a uniform quality and is free from defects is formed. SOLUTION: Isocyanate is reacted with a microcapsule wherein a coloring material is sealed in a wall film made of a heat resistant synthetic resin to activate the surface layer of the capsule, and then the surface layer of the capsule is covered with a wax. Since a hydroxyl group present in the surface layer is turned to a carbamate by the isocyanate so that the surface is made lipophilic, the uniform wax outer wall having a uniform deposition is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microcapsule having a double-walled film formed by forming a wax outer wall on a surface of a microcapsule having a wall film made of a heat-resistant synthetic resin. The present invention relates to a double-walled microcapsule having a uniform and uniform wax outer wall which can be suitably used, and an image recording sheet using the same.

[0002]

2. Description of the Related Art Conventionally, a pressure-sensitive thermal recording system has been proposed as a high-resolution recording system. The pressure-sensitive thermal recording method is a method in which a recording paper coated with microcapsules enclosing a coloring material is heated and pressed by a thermal head to destroy the microcapsules and fix the coloring material on the recording paper to form an image. It is.

[0003] The thickness and material of the wall film are controlled so that the microcapsules used for pressure-sensitive thermal recording are heated at a predetermined temperature or higher and crushed only when pressed at a predetermined burst pressure or higher. . For example, as shown in FIG. 1, microcapsules corresponding to a plurality of colors such as cyan, magenta, and yellow, which are designed so that the breaking temperature / breaking pressure regions of the microcapsules for each color do not overlap, are selectively used for each color. By controlling the pressure and the heating, it is possible to form a high-precision full-color image.

In such an application, as shown in FIG. 2, the surface layer of the microcapsule enclosing the color former 11 by the heat-resistant synthetic resin wall film 12 has an appropriate melting point.
An effective method is to form the outer wall 13 having excellent pressure resistance and solvent resistance to form a double-walled microcapsule. That is, the double-walled microcapsules can control the melting point by selecting the type of the outer wall material having a different melting point, and can control the breaking pressure by controlling the thickness of the inner wall. As a material for forming such an outer wall, waxes having a property of melting when heated but solidifying when cooled are suitable. That is, by forming the double-walled film, the breaking temperature can be controlled on the outer wall of the capsule and the breaking pressure can be controlled on the inner wall of the capsule. Therefore, the breaking condition of the capsule can be accurately controlled for two parameters of temperature and pressure.

As a method of forming a wax outer wall on the capsule surface to form a double wall film, a phase separation method (melting dispersion cooling method) is generally used. According to the phase separation method, first, wax is dispersed in a solvent, and the wax is heated to a temperature higher than the melting point of the wax and well stirred to dissolve the wax. The microcapsules are put into this, and gradually cooled to deposit wax around the microcapsules, thereby forming an outer wall.

However, in the conventional phase separation method, since the affinity between the capsule wall material and the wax is not high, the obtained wax outer wall film is not homogeneous, and the wax is only partially deposited on the capsule surface, or the wax is deposited on the capsule surface. It has been confirmed by the inventors' experiments that horn-like precipitation occurs. That is, in the pressure-sensitive thermal recording, the wax needs to be uniformly coated in order to accurately melt the outer wall of the wax at a predetermined temperature, but in the related art, a uniform double-walled film that can withstand practical use is required. The fact was that they could not be obtained.

[0007] Accordingly, an object of the present invention is to provide a double-walled microcapsule in which a uniform and defect-free wax outer wall is formed on the surface of a microcapsule having a heat-resistant synthetic resin wall film.

[Means for Solving the Problems]

In view of the above problems, as a result of intensive studies, the present inventors have found that a microcapsule in which a coloring material is encapsulated by a wall film made of a heat-resistant synthetic resin is provided with an isocyanate [R-N = C = O].
(R is a hydrocarbon group)] to reactivate the capsule surface and then coat the capsule surface with wax to form a uniform and defect-free wax outer wall, and arrived at the present invention. .

That is, the double-walled microcapsule of the present invention is characterized in that the surface of the capsule is activated by pretreatment with an isocyanate before the wax is deposited. According to the conventional phase separation method, a uniform wax outer wall film could not be obtained because the affinity between the capsule wall material and the wax was not high. However, according to the present invention, as shown in the schematic diagram of FIG. Hydroxyl groups and other hydrophilic groups present on the surface are isocyanates (phenyl isocyanate in the figure)
It is considered that the surface of the microcapsules becomes lipophilic due to the reaction and the surface characteristics become uniform, and a uniform wax outer wall having no unevenness in the precipitated state is formed.

The isocyanate used in the present invention includes:
Aliphatic or aromatic isocyanates are preferred.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION [I] Double-walled microcapsule The double-walled microcapsule of the present invention is a microcapsule in which a coloring material is encapsulated by a wall film made of a heat-resistant synthetic resin (hereinafter referred to as a raw material microcapsule). After the capsule surface is activated by reacting an isocyanate with the isocyanate, wax is precipitated to form a double-walled film. Since the capsule surface is activated (carbamated) with isocyanate before the wax is deposited, there is almost no non-double-walled film, and a uniform wax outer wall without unevenness in the deposited state It is formed. Therefore, it can be suitably used for applications requiring the accuracy of the capsule breaking conditions (such as pressure-sensitive thermal recording).

Hereinafter, the double-walled microcapsule of the present invention will be described in detail.

[A] Raw material microcapsules The raw material microcapsules used in the present invention have a wall film made of a heat-resistant synthetic resin, and are not destroyed by pressure or heating generated during storage or transportation before use. The material can be held sufficiently, but is destroyed when pressurized to a predetermined pressure or higher, and the color material (ink, color former, etc.) contained inside
Can be released.

The particle size of the raw material microcapsules is 0.1 to 100.
μm, preferably 1-20 μm.

The heat-resistant synthetic resin constituting the wall film (the inner wall of the double wall film) of the raw material microcapsule may be either a thermosetting resin or a high melting point thermoplastic resin.
Examples include melamine formaldehyde polymers, urea formaldehyde polymers, polyureas, polyurethanes, polyamides, polyimides, polyesters and the like. Preferred are polymers comprising melamine formaldehyde polymer, urea formaldehyde polymer and both. Microcapsules having wall films made of heat-resistant synthetic resin are known in situ methods, interfacial polymerization methods,
It can be formed by a coacervation method or the like.

For example, the in si described in JP-A-58-82785
When producing microcapsules using the tu method, first,
An aqueous mixture containing at least a water-soluble cationic urea resin and an anionic surfactant is prepared. Separately, a color former solution (capsule core substance) is prepared and mixed with the above-mentioned aqueous mixed solution. This is emulsified and dispersed by using a homogenizer, a stirrer, an ultrasonic wave or the like so that the color former solution becomes fine droplets of several μm. Color former solution is a non-volatile organic solvent that is hardly soluble or insoluble in water (for example, phosphate ester, phthalate ester, fatty acid amide,
The coloring material is prepared by dissolving or dispersing the coloring material in an alkylated biphenyl, an alkylated naphthalene, a diarylethane, etc.) and / or a volatile organic solvent (for example, toluene, benzene, xylene, ketone, ester, etc.). As a coloring material, in addition to an ink that already exhibits a certain color, a coloring system used in a conventional thermosensitive recording material (for example, a combination of a leuco coloring agent and a developing agent, a combination of a diazo compound and a coupler, and the like). Can be used.

To this, a prepolymer produced from melamine, thiourea, formaldehyde or the like is added as a raw material for the microcapsule wall material. The prepolymer may be present in the mixed solution before emulsification in advance, or may be added in several portions in the middle. While gently stirring the dispersion containing the prepolymer,
The microencapsulation is completed by reacting at 2.5 to 6.0 at a reaction temperature of 15 to 60 ° C for 2 to 15 hours. During the course of the reaction, an appropriate amount of water can be added.

In the encapsulation, the amount of the prepolymer used is preferably in the range of 0.1 to 1 g per 1 g of the color former solution.

Examples of the anionic surfactant include fatty acid salts, higher alcohol sulfates, and alkyl allyl sulfonates, but sodium dodecylbenzene sulfonate is preferred.

[B] Formation of double-walled microcapsules (1) Activation treatment of capsule surface The double-walled microcapsules of the present invention use isocyanate before being double-walled with wax. Characterized in that the capsule surface is activated.

As a result, hydrophilic groups (for example, hydroxyl groups derived from methylolmelamine) present on the surface of the raw material microcapsules are blocked by the isocyanate, and the surface of the capsule is made lipophilic to make the surface characteristics uniform. Thus, a uniform wax outer wall can be formed.

The isocyanate is represented by the general formula RN = C = O. The substituent R is a hydrocarbon group, including aliphatic (which may be linear or branched, and also includes alicyclic and crosslinked cyclic) and aromatic (including polycyclic (condensed ring and ring assembly)). ) May be used. As the isocyanate, C 1 -C 1
An aliphatic isocyanate having 20 or an aromatic isocyanate having 6 to 20 carbon atoms is preferable, and an aliphatic isocyanate having 3 to 10 carbon atoms or aromatic isocyanate having 6 to 10 carbon atoms is more preferable. Specific examples of isocyanates that can be preferably used in the present invention include methyl isocyanate, ethyl isocyanate, vinyl isocyanate, propyl isocyanate, isopropenyl isocyanate, butyl isocyanate, phenyl isocyanate, tolyl isocyanate, and nitrophenyl isocyanate. And naphthyl isocyanate, toluene diisocyanate, diphenylmethane diisocyanate and the like. Particularly preferred among these are phenyl isocyanate, toluene diisocyanate and diphenylmethane diisocyanate.

The above isocyanate is added to microcapsules 10
0 to 100 parts by weight, using 10 to 100 parts by weight, in a solvent, 30 to
By reacting at 80 ° C. for about 3 to 5 hours, the capsule surface is made lipophilic (carbamate). As the solvent, 2,6-dimethyl-4-heptanone or the like can be used.

Before reacting the isocyanate, it is preferable to sufficiently wash the raw material microcapsules to remove dyes and the like adhering to the capsule surface. If necessary, a catalyst such as an organometallic compound or an amine-based substance may be added.

(2) Formation of Wax Outer Wall Next, wax is deposited on the surface-activated raw material microcapsules by a phase separation method to form a double wall film. First, the wax is dispersed in a solvent that is not miscible with the wax at room temperature (20 to 30 ° C.) and is miscible at a temperature equal to or higher than the melting point of the wax, and heated to a temperature equal to or higher than the melting point of the wax to dissolve the wax. At this time, the wax is thoroughly stirred to completely dissolve the wax, and then the microcapsules are charged and dispersed therein, and gradually cooled to deposit wax around the microcapsules, thereby forming a wax outer wall.

Any known wax can be used without particular limitation so long as it is hydrophobic and has a distinct melting point. Examples thereof include paraffin, microcrystalline, carnauba, polyolefin, montan, and candelilla. And waxes such as beeswax can be used.

In the present invention, the breaking temperature of the microcapsules can be adjusted to an arbitrary temperature by selecting the melting point of the wax. The melting point of the wax varies depending on the application, but when used for pressure-sensitive heat-sensitive recording, the melting point is generally 60 to 300.
It is preferred to use a wax at a temperature of ° C.

The wax preferably has an acid value (mg of potassium hydroxide added per g of wax) of 3 or more. For example, Mitsui Petrochemical Industries Ltd. Mitsui High Wax 405MP, 300MP, 32MP
0MP and the like.

[II] Image Recording Sheet The image recording sheet of the present invention reacts with a double-walled microcapsule of the present invention, a binder resin, and a coloring material (eg, leuco dye) inside the capsule, on a support. It is manufactured by applying a coating solution containing various additives such as a color developer, a sensitizer, a reaction accelerator and the like to form at least one color-forming layer.

As the support, any paper support used for ordinary thermal paper can be used, and plastic film laminated paper, synthetic paper, plastic film and the like can be used. A back coat layer may be provided to correct the curl balance of the support or to improve the chemical resistance from the back surface, or to form a label by combining release paper with an adhesive layer on the back surface. You may.

The developer, sensitizer and the like are solid substances at ordinary temperature, and are usually added to the coating liquid after being atomized by a grinding machine such as a ball mill, an attriser, a sand grinder or an appropriate emulsifying device.

After adjusting the coating solution to a concentration suitable for coating,
Coating is performed on the support by a known applicator method, bar coater method, roll coating method, spray coating method, die coating method, lip coating method, air knife coating method, or the like. The coating amount is 0.5 to 20 g / m ² in terms of the dry weight of the solid.

The coloring layer of the image recording sheet of the present invention is selectively heated according to a desired image pattern to raise the temperature to a predetermined temperature or higher, and the coloring layer is pressed to deform the microcapsule wall film. An image can be formed on the sheet by discharging the color material inside.

A color image can be recorded using the image recording sheet of the present invention. Enclose multiple types of color materials in raw material microcapsules with different breaking pressures for each color,
Further, by forming a double-walled film with a wax having a different melting point for each color, each microcapsule is deformed at a different temperature and pressure for each color to release the internal coloring material. A color forming layer containing capsules of a plurality of colors is formed on a support to form a sheet, and the color forming layer is deformed at a different temperature and a different pressure for each color so that only a capsule of a specific color is deformed to release a color material inside. What is necessary is just to heat and press selectively.

Therefore, when the color-forming layer is formed by using microcapsules containing, for example, cyan, magenta, and yellow coloring materials, when the capsule layer is heated and pressed at a temperature and pressure corresponding to a certain color (for example, cyan). The capsule of that color (cyan) is crushed and a color material comes out, but other colors (for example, magenta and yellow) are not crushed. Therefore, sequentially by color (for example, cyan, magenta, yellow)
By heating and pressurizing at a predetermined temperature and pressure, a color image can be formed easily and at high speed.

For example, as shown in FIG. 1, microcapsules of cyan, magenta and yellow are simultaneously heated and
Even when the pressure is applied, when the temperature and pressure are in the region A, the cyan microcapsules are crushed, but the magenta and yellow microcapsules are not crushed. Similarly, in region B,
The magenta microcapsules collapse, but the cyan and yellow microcapsules do not. In the area C,
The yellow microcapsules collapse, but the cyan and magenta microcapsules do not. By selectively applying the temperature and pressure between the regions A to C in this manner,
In the area A, cyan can be emitted, in the area B, magenta can be emitted, and in the area C, yellow can be emitted.
Each image of cyan, magenta, and yellow is sequentially formed, and a color image can be formed.

In the image recording sheet of the present invention, since the capsule is uniformly formed into a double-walled film, the breaking temperature can be controlled by the outer wall of the capsule and the breaking pressure can be controlled by the inner wall of the capsule. That is, since the breaking condition of the capsule can be accurately controlled for the two parameters of temperature and pressure, it is easy to design so that the breaking temperature and breaking pressure regions (regions A to C) of the microcapsules for each color do not overlap. Become.

The image recording sheet of the present invention can be used for various applications such as facsimile paper, printer paper, labels, price tags, and tickets.

[0039]

The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto.

Example 1 (1) Preparation of Raw Material Microcapsules Raw material microcapsules having a particle size of 5 to 8 μm covered with a melamine resin / urea resin were prepared by an in situ method.

(2) Washing of microcapsules 0.4 g of the above-mentioned raw material microcapsules were washed with 2,6-dimethyl-4-heptanone (20 ml), sedimented, and the microcapsules were separated.

(3) Carbamate formation The microcapsules separated in (2) were dispersed in 2,6-dimethyl-4-heptanone (20 ml), and phenyl isocyanate was added in 0.1 ml.
4 ml was added, and the mixture was heated with stirring at 60 ° C. for 5 hours to activate the capsule surface.

(4) Change of solvent Trichloroethylene was converted from 2,6-dimethyl-4-heptanone as a solvent to obtain a capsule dispersed state.

(5) Formation of double wall film While the solution was kept at a temperature of 83 ° C. and well stirred, a wax (Mitsui High Wax 405MP, Mitsui Petrochemical Industry Co., Ltd.)
Was mixed and stirred for 30 minutes. In this state, the temperature was gradually lowered to form a double-walled microcapsule.

Observation of the obtained microcapsules having a double wall film with a microscope revealed that there was no unevenness in the deposition state of the wax, and that there was almost no non-double wall film microcapsules.

Comparative Example 1 In Example 1, except that the processing of (3) was not performed,
Similarly, a double-walled microcapsule was prepared.

Observation of the obtained microcapsules having a double wall film with a microscope revealed that there was unevenness in the deposition state of the wax, some of which were deposited in a square shape, and many of which did not have a double wall film. I was seen.

[0048]

As described in detail above, the double-walled microcapsules of the present invention are obtained by reacting isocyanate to activate the capsule surface and then coating the raw material microcapsule surface with wax. Therefore, a uniform and defect-free wax outer wall can be formed. Since the double-walled microcapsule of the present invention is heated above the melting point of the wax outer wall, and is broken only when pressed above the breaking pressure of the heat-resistant synthetic resin inner wall, the breaking accuracy of the capsule is reduced. It is extremely useful for required applications (such as pressure-sensitive thermal recording).

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between pressure and temperature at which a capsule is crushed.

FIG. 2 is a cross-sectional view of a double-walled microcapsule.

FIG. 3 is a schematic diagram showing activation of a capsule surface.

[Explanation of symbols]

 1: Double-walled microcapsules 11: Color homer 12: Heat-resistant synthetic resin wall 13: Wax outer wall

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B01J 13/16 B41M 5/18 112 C09D 7/12 B01J 13/02 A 201/00 D (72) Inventor Kazuyuki Shinbo 2-36-9 Maeno-cho, Itabashi-ku, Tokyo F-term in Asahi Kogaku Kogyo Co., Ltd. BB06 BB12 BB15 BB25 DB01Z DC46W DD05Z DD08Z DD10Y DD34Z DD35Z EA08 4J038 PA18 PB11 PC08 PC10

Claims (4)

[Claims] 1. A microcapsule in which a coloring material is encapsulated with a wall film made of a heat-resistant synthetic resin is reacted with isocyanate to activate the capsule surface, and then the capsule surface is coated with wax to form an outer wall. A double-walled microcapsule characterized by forming: 2. The double-walled microcapsule according to claim 1, wherein the isocyanate is an aliphatic isocyanate or an aromatic isocyanate. 3. The double-walled microcapsule according to claim 1, wherein the heat-resistant synthetic resin forming the wall film of the microcapsule is made of a melamine resin and / or a urea resin. Double-walled microcapsules. 4. An image recording sheet having a support and at least one color-forming layer, wherein the color-forming layer comprises
An image recording sheet containing the double-walled microcapsule according to any one of claims 1 to 3.