JPH048238B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a heat-sensitive recording sheet, and particularly to a method for producing a heat-sensitive recording sheet with good thermal responsiveness and capable of high-speed recording.

Thermosensitive recording sheets record images by utilizing physical and chemical changes in substances caused by thermal energy, and a large number of processes have been studied so far.

So-called wax-type heat-sensitive recording sheets have been around for a long time, and are used for electrocardiograms, etc., as they utilize the physical changes in substances caused by heat. In addition, those that utilize chemical changes due to heat,
Various coloring mechanisms have been proposed, but
In particular, what is called a two-component color-forming heat-sensitive recording sheet is typical.

Two-component color-forming thermosensitive recording sheets are made by dispersing two types of heat-reactive compounds in the form of fine particles, mixing them with a binder, etc., and supporting the two types of heat-reactive compounds so that they are separated from each other by the binder. It is applied to the body, and records are obtained by utilizing the coloring reaction that occurs when one or both of them melt and come into contact with each other by heating. These two types of heat-reactive compounds are generally called electron-donating compounds and electron-accepting compounds, and although there are many types of combinations, they can be broadly categorized to form images of metal compounds. There are two types: those that form dye images, and those that form dye images.

Some methods for forming images of metal compounds use phenols and other organic reducing agents, xylate agents, sulfur compounds, and amino compounds as electron-donating compounds, and organometallic salts as electron-accepting compounds. These two react by heating to produce metals, metal complex compounds, metal sulfides, etc., giving colored images. Specifically, the combination of thiourea and heavy metal salts (US Patent No. 2740895), the combination of gallic acid etc. and stearic acid metal salts (US Patent No. 2740895), etc.
2663654-7), a combination of hydroquinone and silver behenate (US Pat. No. 3,031,329), and a combination of hexamethylenetelamine and a tin compound (US Pat. No. 2,813,043).

On the other hand, there are methods that use an electron-donating colorless dye as an electron-donating compound and phenols or other acidic substances as an electron-accepting compound to form a dye image (Japanese Patent Publication No. 43-4160,
44−3680).

Such heat-sensitive recording sheets can be coated in a thin layer at a coating speed of 150 m/min or more using relatively simple equipment or operations. It was manufactured using a reverse roll coating method.

These two-component color-forming heat-sensitive recording sheets (1) are primary color-forming and do not require development; It has many advantages as a recording sheet, such as (2) the paper quality is similar to regular paper, and (3) ease of handling. In particular, those using colorless dyes as electron-donating compounds have further advantages such as (4) high color density, and (5) easy production of heat-sensitive recording sheets with various color hues, and have great utility value. Therefore, it is most commonly used as a heat-sensitive recording material.

Thermosensitive recording sheets having the above-mentioned major features have recently been attracting attention as image-receiving recording paper for facsimile communications.

When heat-sensitive recording sheets are used as facsimile recording paper, there is no need for a developing process, which simplifies the equipment configuration, and there are no consumables required other than the recording paper, so there are advantages in terms of maintenance. There are also many. However, since it is a thermal recording method, it also has drawbacks. That is, the recording speed is slow.
These are thought to be mainly due to the poor thermal responsiveness of the thermal recording head and the thermal recording sheet. recently,
As for thermal recording heads, those with good thermal responsiveness are being developed, but as for thermal recording sheets, there is still no one that can sufficiently respond to the problem, and the development of one has been desired.

Therefore, an object of the present invention is to provide a heat-sensitive recording sheet with good thermal responsiveness. Specifically, the object of the present invention is to provide a heat-sensitive recording sheet that can perform sufficient recording using heat pulses of 1 ms or less.

The purpose of the present invention is to coat a support with a coating liquid containing a white or light-colored pigment with an oil absorption of 75 ml/100 g or more as specified in JIS K5101, and a coating liquid containing an electron-donating colorless dye and an electron-accepting compound as main components. of,
This is achieved by impinging and coating the support to be coated with superimposed free-falling curtains, each of which has a heat-sensitive recording layer as the upper layer, containing an electron-donating colorless dye and an electron-accepting compound as main components. Ta.

An example of the structure of a conventional heat-sensitive recording sheet is that an electron-donating colorless dye (hereinafter referred to as a color former) and an electron-accepting compound (hereinafter referred to as a color developer) are dispersed in a binder such as polyvinyl alcohol. Examples include those coated on a support. At least one of these melts when heated and the two react to form a recorded image, but if left as is, there are various practical problems. One is dirt on the recording head during recording. In order to prevent this, oil-absorbing substances such as inorganic pigments are dispersed in the binder, and fatty acid amides, fatty acids, metal soaps, etc. are added to improve mold release properties from the head. . Therefore, in addition to color formers and color developers that directly contribute to color development, pigments, waxes, additives, etc. in several times the amount are coated on the support.
A thermosensitive recording sheet will be constructed.

This is very disadvantageous in terms of the energy required for recording. One is that the color-forming component is extensively diluted, reducing its utilization and requiring more color former and developer to obtain the same density record. Next, as the amount of coating increases, the heat capacity of the coloring layer increases, so even if the same amount of thermal energy is applied, the temperature of the coloring layer does not rise and sufficient recording density cannot be obtained, resulting in a large amount of thermal energy being used. It will take a while. In order to compensate for this drawback and increase the utilization efficiency of the coloring layer, we have created an oil-absorbing layer made of highly oil-absorbing pigments on the support, and a heat-sensitive coloring layer containing a coloring agent and a color developer as main components on top of the oil-absorbing layer. We have already proposed a heat-sensitive recording sheet.
A common manufacturing method is to first coat an oil-absorbing pigment layer on a support sheet, and then provide a heat-sensitive coloring layer thereon.

However, this method is not only uneconomical in that it requires passing through the coating process and drying process twice in order to produce a heat-sensitive recording sheet, but also is expensive when coating the heat-sensitive recording layer. Various problems arise because the layer containing oil-absorbing pigment as a main component is coated and dried.

First, when applying a thin layer of heat-sensitive coating liquid, the heat-sensitive coating liquid rapidly penetrates into the highly oil-absorbing pigment layer, so it is necessary to apply the coating liquid in advance using conventional methods such as air knives, blades, and bars. With coating methods that apply the product and then scrape off the excess, and methods that adjust roll clearance such as the reverse roll method,
Difficult to control uniform coating amount.

A solution to these problems is to dilute the heat-sensitive coating solution, but this method not only makes it easier for the heat-sensitive coating solution to penetrate into the pigment layer, but also has drawbacks such as increased drying load.

Second, because the heat-sensitive coating solution penetrates into the highly oil-absorbing pigment layer, the interface between the two layers cannot always be clearly separated, making it impossible to achieve the purpose of forming a heat-sensitive coloring layer on the highly oil-absorbing pigment layer. . As a result, sufficient color density cannot be obtained.

Furthermore, the third problem is that when the heat-sensitive coating liquid components permeate into the highly oil-absorbing pigment layer, molecularly dispersed materials such as binders permeate selectively, so the composition in the thickness direction of the heat-sensitive recording layer changes. It is not uniform, and there are disadvantages such as powder falling easily on surface areas where the amount of binder is low.

Therefore, the present inventors have conducted intensive studies on a method for manufacturing a heat-sensitive recording sheet that can shorten the coating and drying steps, and that can effectively separate the heat-sensitive recording layer and the layer mainly containing pigments to record with less thermal energy. By colliding with a support a free-falling curtain in which a coating liquid mainly containing a highly pigmented pigment and a coating liquid mainly containing a color forming agent and a color developer are superimposed, an oil-absorbing layer is formed on the support. It has been found that it is possible to simultaneously form a layer mainly containing a high pigment content and a heat-sensitive recording layer thereon, thereby producing a heat-sensitive sheet with good separation between the two layers and sufficient color density.

According to the method of the present invention, since the highly oil-absorbing pigment coating liquid and the heat-sensitive coating liquid are applied and dried simultaneously, the materials contained in each layer are less likely to move, and the highly oil-absorbing pigment is uniformly separated after drying. A layer and a heat-sensitive recording layer are obtained. For this reason, the recording sheet of the present invention has better responsiveness to very short heat pulses than a method in which a highly oil-absorbing pigment layer and a heat-sensitive recording layer are applied sequentially.
Sufficient color density can be obtained with a small amount of thermal energy. The reason for this is that the upper layer of the recording layer is formed with a heat-sensitive recording layer containing a color forming agent and a color developer as main components, and is separated from pigments with a large heat capacity, so that most of the heat generated in the thermal head is absorbed. This is thought to be because it contributes to color development. Furthermore, the heat-sensitive recording sheet of the present invention hardly causes staining of the thermal head or adhesion to the thermal head. The reason for this is thought to be that the pigment layer located directly under the heat-sensitive recording layer has high oil absorption, so the color former and developer melted by the heat of the thermal head are quickly absorbed into the pigment coating layer. It will be done. What is also noteworthy is that since this absorption mainly occurs at the interface between the recording layer and the oil-absorbing pigment layer, a color former is formed on the surface of the pigment layer, resulting in a very high reflection density. Conceivable. Furthermore, perhaps because the pigment layer acts as a heat insulator, the color density during short heat pulses was much higher than expected compared to the previously known single-layer coating.

Paper is generally used as the support, but synthetic paper, plastic film, etc. can be used if necessary.

The oil-absorbing pigment layer provided on the support is
Oil absorption is 75 according to the measurement method specified in JIS K5101.
ml/100g or more is fine, but 100ml/100g or more is fine.
It is desirable that the weight is 100g or more. In addition, it is desirable that the volume average particle diameter of the pigment is 5.0 μm or less,
Coated paper with high smoothness can be obtained.

Specifically, oil absorbers selected from kaolin, calcined kaolin, talc, waxite, diatomaceous earth, calcium carbonate, aluminum hydroxide, magnesium hydroxide, magnesium carbonate, titanium oxide, barium carbonate, urea-formalin filler, cellulose filler, etc. Items with a concentration of 75ml/100g or higher are listed. The oil absorption of these pigments may be increased by surface treatment or the like. Pigment coating layer is 1 to 15
g/m ^{2 ,} preferably 2 to 7 g/m ² .

The heat-sensitive coloring layer provided on the pigment coating layer mainly contains a color former and a color developer, but waxes, metal soaps, ultraviolet absorbers, etc. are added as necessary. Furthermore, there is no problem even if the above-mentioned pigments are mixed. A high color density can be obtained by increasing the coating layer of the thermosensitive coloring layer, but this also causes phenomena such as staining of the recording head and staining due to trailing during high-speed recording, and the thickness of the thermosensitive coloring layer is
It is desirable that the thickness be 7 μm or less, more preferably 5 μm or less. The lower limit is determined by the color density, but 2 μm or more is required. In addition, in order to obtain high-density recording during high-speed recording, it is necessary that the color-forming component in the thermosensitive color-forming layer, that is, the sum of the color former and the color developer, be sufficiently large compared to other additives, etc. Preferably, it needs to account for 50% or more, more preferably 65% or more of the total solid content excluding the binder.

The so-called curtain coating method, in which a free-falling curtain of coating liquid is formed and applied by colliding with a support, has already been disclosed in Japanese Patent Publications Nos. 49-24133 and 49-49.
It is disclosed in No. 35447, Japanese Patent Application Laid-Open No. 54-74762, etc., and is known to be suitable for coating high viscosity coating liquids, thin layer coating, and high speed coating.

The color forming agent used in the present invention is not particularly limited as long as it is used in general pressure-sensitive recording paper, heat-sensitive recording paper, etc. To give a specific example
(1) 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (crystal violet) as a triarylmethane compound.
lactone) 3,3-bis(p-dimethylaminophenyl)phthalide, 3-(p-dimethylaminophenyl)-3-(1,2-dimethylindole-3
-yl)phthalide, 3-(p-dimethylaminophenyl)-3-(2-methylindol-3-yl)phthalide, 3-(p-dimethylaminophenyl)-3-(2-phenylindole- 3-il)
Phthalide, 3,3-bis-(1,2-dimethylindol-3-yl)-5-dimethylaminophthalide, 3,3-bis-(1,2-dimethylindol-3-yl)-6-dimethyl aminophthalide,
3,3-bis-(9-ethylcarbazole-3-
yl)-5-dimethylaminophthalide, 3,3-
Bis-(2-phenylindol-3-yl)-5
-dimethylaminophthalide, 3-p-dimethylaminophenyl-3-(1-methylpyrrole-2-
yl)-6-dimethyl-aminophthalide, etc.: (2) As a diphenylmethane compound, 4,4'-bis-
Dimethylaminobenzhydrin benzyl ether, N-halophenyl leukoolamine, N-
2,4,5-trichlorophenylleucoolamine, etc.: (3) As xanthene compounds, rhodamine B-anilinolactam, rhodamine B-p-nitroanilinolactam, rhodamine B-p-chloroanilinolactam, 3 -diethylamino-7-dibenzylaminofluorane, 3-diethylamino-7-octylaminofluorane, 3-diethylamino-7-phenylfluorane, 3-diethylamino-7-3,4-dichloroanilinofluorane, 3-diethylamino-7-(2-chloroanilino)fluorane, 3-diethylamino-6-methyl-7-anilinofluorane, 3-piperidino-6-methyl-7-anilinofluorane, 3-ethyl-tolylamino-6 -Methyl-7-anilinofluoran, 3-ethyl-tolylamino-6-methyl-7-phenylfluoran, 3-diethylamino-7-(4-nitroanilino)fluoran, etc.: (4) As a thiazine compound, benzoyl Leucomethylene blue, p-nitrobenzoyl leucomethylene blue, etc.: (5) as a spiro compound, 3
-Methyl-spiro-dinaphthopyran, 3-ethyl-spiro-dinaphthopyran, 3,3'-dichloro-
Spiro-dinaphthopyran, 3-benzyl spiro-
dinaphthopyran, 3-methylnaphtho-(3-methoxy-benzo)-spiropyran, 3-propyl-
Examples include spiro-dibenzopyran and mixtures thereof. These are determined by the application and desired properties.

As the color developer used in the present invention, phenol derivatives and aromatic carboxylic acid derivatives are preferable.
Particularly preferred are bisphenols. in particular,
As phenols, p-octylphenol,
p-tert-butylphenol, p-phenylphenol, 1,1-bis(p-hydroxyphenyl)propane, 2,2-bis(p-hydroxyphenyl)propane, 1,1-bis(p-hydroxy phenyl)pentane, 1,1-bis(p-hydroxyphenyl)hexane, 2,2-bis(p-
Examples include hydroxyphenyl)hexane, 1,1-bis(p-hydroxyphenyl)-2-ethyl-hexane, and 2,2-bis(4-hydroxy-3,5-dichlorophenyl)propane.

Aromatic carboxylic acid derivatives include p-hydroxybenzoic acid, ethyl p-hydroxybenzoate,
Butyl p-hydroxybenzoate, 3,5-di-
Examples of tert-butylsalicylic acid, 3,5-di-α-methylbenzylsalicylic acid and carboxylic acids include polyvalent metal salts thereof.

Examples of waxes include paraffin wax, cownabar wax, microcrystalline wax, polyethylene wax, and higher fatty acid amides such as stearic acid amide, ethylene bisstearamide, and higher fatty acid esters.

Examples of the metal soap include polyvalent metal salts of higher fatty acids, such as zinc stearate, aluminum stearate, calcium stearate, and zinc oleate.

These are dispersed and applied in a binder. Binders are generally water-soluble, such as polyvinyl alcohol, hydroxyethyl cellulose, hydroxypropyl cellulose, ethylene-maleic anhydride copolymer, styrene-maleic anhydride copolymer, isobutylene-maleic anhydride copolymer. Examples include polyacrylates, polyacrylic acids, starch derivatives, casein, and gelatin. In addition, water-resistant agents (gelling agents, cross-linking agents) are added to these binders for the purpose of imparting water resistance, and emulsions of hydrophobic polymers, specifically styrene-
Butadiene rubber latex, acrylic resin emulsion, etc. can also be added.

In the present invention, the coating liquid contains a film-forming substance. Such a film-forming substance is generally used as a binder, and the same film-forming substance may be used in the heat-sensitive recording layer and the layer mainly containing pigment.

For example, film-forming substances include ethyl cellulose,
Methylcellulose, carboxymethylcellulose, starches, hydroxyethylcellulose,
Natural polymeric substances such as cellulose nitrate and styrene-maleic anhydride copolymers, isobutylene-
Synthetic polymer materials such as maleic anhydride copolymer, polyvinyl alcohol, styrene-butadiene rubber latex, vinyl acetate latex, acrylonitrile-butadiene rubber latex, methyl acrylate-butadiene rubber latex, etc. (these may be used alone or in combination) ).
The film-forming substance may be water-soluble or oil-soluble, but it is preferable to use one that can be prepared as an aqueous coating liquid in view of the manufacturing process.

Specific examples are shown below, but it goes without saying that the present invention is not limited thereto.

Example 1 4 kg of calcined kaolin (oil absorption 150 ml/100 g) was added to 6 kg of water and dispersed with a homogenizer for 5 minutes.
8 kg of 12.5% polyvinyl alcohol (degree of polymerization: 1000, degree of saponification: 99%) was added and thoroughly mixed to obtain liquid A. The concentration of liquid A is 27.8%, and the viscosity at room temperature is
It was 35 cp.

Crystal Violet Lactone 0.7Kg 5%
The mixture was poured into 4 kg of polyvinyl alcohol, and ground and dispersed in a ball mill to obtain a desired particle size distribution to obtain liquid B.

Furthermore, 2 kg of bisphenol A and 2 kg of stearic acid amide were added to 20 kg of 5% polyvinyl alcohol, and ground and dispersed to the desired particle size distribution using a ball mill.
It was made into liquid C. Add liquid B and liquid C, add 3.5 kg of 20% zinc stearate dispersion, and mix thoroughly.
It was made into liquid D. The concentration of Solution D was 20.5%, and the viscosity of Solution D at room temperature was 37 cp.

The coating liquids A and D thus obtained were spread over a width of 200 mm.
The coating was performed using an extrusion type curtain coating device having a slit width of 0.4 mm and a slit width of 0.4 mm. A liquid,
Liquid D is supplied to each supply head to form a superimposed free-falling curtain of liquids A and D2 with a height of 10 cm, and a high quality material with a width of 180 mm and a basis weight of 50 g/m ² is continuously running at 180 m/min. Liquid D was coated on top so that the solid content of the upper layer was 3 g/m ² and the lower layer was 5 g/m ² and dried in a conventional manner. A thermal recording sheet was obtained by calendering at 20 kg/cm. When the cross section of the coating film of the heat-sensitive recording sheet thus obtained was observed under a microscope, very good interlayer separation was observed. The thickness of the heat-sensitive recording layer was approximately 2.6 μm.

A thermal recording head was brought into close contact with this recording paper under a pressure of 500 g/cm ² and a current pulse of 0.8 ms was applied to it for 30 minutes.
When recording was performed by applying electrical energy of mJ/mm ² (head temperature rises to 330°C when not in contact with recording paper), the recording density was 1.02.
(reflection density at 610 nm) was obtained. Further, no staining or adhesion of the head occurred.

Comparative Example 1 Using the same method as in Example 1, liquid A was added to Example 1.
Solid content coating amount 5 on high-quality paper with a basis weight of 50 g
g/m ² using an air knife coater and dried in a conventional manner to obtain a base paper. No problems occurred with the application of liquid A. D to this
The liquid was coated using an air knife coater so that the solid content coating amount was 3 g/m ² . In this case, the concentration of liquid D is
It was necessary to dilute it to 12%. After application 20KgW/
A recording sheet was obtained by calendering at a pressure of cm. When the cross section of the coating film was observed under a microscope, it was not possible to distinguish between the heat-sensitive recording layer and the pigment-containing layer as clearly as in Example 1.

When a thermal recording head was brought into close contact with this recording paper under a pressure of 500 g/cm ² and a current pulse of 0.8 ms was applied in the same manner, recording was performed, resulting in a recording density of 0.85 (610 nm).
reflection density) was obtained. In order to obtain the concentration of 1.02 obtained in Example 1 of the present invention, it is necessary to widen the current pulse width to 0.95 ms, which is approximately 20%
It was found that it was necessary to increase the recording energy. No staining or adhesion of the head occurred.

Comparative Example 2 A thermosensitive recording sheet was bloomed in the same manner as in Comparative Example 1, except that Liquid D was applied using a Mayer bar (bar No. 6). As a result, bar streaks were generated due to seepage, and fogging occurred in these bar streaks.

The D solution was diluted to 10%, coated with a bar No. 10, dried, and calendered to obtain a heat-sensitive recording sheet having a solid coating weight of 3 g/m ² .

As a result, when a thermal recording test was conducted in the same manner as in Comparative Example 1, the concentration with a 0.8 ms electric particle pulse was 0.83 (610n
reflection density at m).

Comparative Example 3 A heat-sensitive recording sheet was prepared in the same manner as in Comparative Example 1, except that Liquid D was applied using a reverse roll coater.

As a result, the liquid D penetrated significantly and could not be applied uniformly.

As mentioned above, by using the method of the present invention, it is not only possible to easily produce a heat-sensitive recording sheet with separated properties in a single application, but also it is possible to record at a higher density than conventional heat-sensitive recording paper during high-speed recording. It became.

Claims (1)

[Claims] 1. On the support, the oil absorption amount specified in JIS.K5101 is 75.
ml/100g or more of a coating layer containing a white or light-colored pigment, and a coating layer further containing an electron-donating colorless dye and an electron-accepting compound as main components on the coating layer. , a coating liquid containing the white or light-colored pigment, and a coating liquid containing as a main component a mixture of the electron-donating colorless dye and the electron-accepting compound dispersed in a binder, respectively, are superimposed. 1. A method for producing a heat-sensitive recording sheet, characterized in that the coating is simultaneously applied by colliding a free-falling curtain onto a support to be coated. 2 The thickness of the coating layer containing as a main component a mixture of an electron-donating colorless dye and an electron-accepting compound dispersed in a binder is
The method for producing a heat-sensitive recording sheet according to claim 1, wherein the thickness is 7 μm or less.