JP2003011487A - Ink jet printing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ink jet printing method capable of printing a lustrous color image without oozing ink at all even when the image is printed on an ordinary paper or a re-produced rough printing paper and capable of obtaining a rigidness (resistance to water and light) in the same level as a conventional printing method employing plate printing ink since the ink jet printing method is suitable for multi-color printing and capable of directly effecting plateless printing from a computer, however, oozing on the ordinary paper can not be prevented in the conventional ink jet printing, inferior in the rigidness (resistance to water and light) of a printed matter and not capable of printing the lustrous color image. SOLUTION: The ink jet printing is effected employing an aqueous ink containing a water-soluble photopolymerizable monomer and water soluble vat dye and, thereafter, UV light or laser is irradiated against ink dots on the surface of a paper to fix the ink.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an ink jet printing method. More specifically, the present invention relates to an ink jet printing method capable of obtaining a glossy image and enabling high-speed printing, and an ink composition used therefor.

[0002]

2. Description of the Related Art The ink jet printing method is a plateless printing method in which characters and images are printed by a large number of ink dots which are ejected from nozzles to eject minute droplets of ink onto the paper surface. In the ink jet printing method, since fine ink droplets are ejected at high speed and with high frequency, it is preferable that the surface tension of the ink is as large as possible and the ink viscosity is as low as possible. Therefore, as the ink for ink jet printing, a water-based ink using water as a solvent is mainly used. This water-based ink basically consists of water colored with a water-soluble dye, and since it is necessary to maintain the ink viscosity as low as possible, it does not include any polymer compound having a property of increasing the ink viscosity, such as a binder. Not not.
This point is significantly different from the conventional plate printing ink.

The ink for ink jet printing is an aqueous solution ink having physical properties similar to those of a fountain pen and showing low viscosity. Therefore, the ink for ink jet printing has a drawback in that a printed image or a character is easily blurred. In the ink jet printing ink, a water-soluble polymer or the like that increases the viscosity cannot be added as a binder, so the problem of ink bleeding is a drawback of the ink jet printing method, which is difficult to solve.

Further, since the water-soluble dye is used for coloring the water-based ink, there is also a problem that the water resistance of the printed matter is poor in the ink jet printing. Since these drawbacks are fatal defects in printing technology, the ink jet printing method is not used as a general printing method.

However, the ink jet printing method is a plateless printing method, has a good compatibility with a digital signal, and has a feature that an image signal of a computer can be printed as it is. Therefore, the ink jet printing method is used as a printer for a computer. Further, the ink jet printing method can easily deal with multicolor ink by increasing the number of nozzles, and is therefore suitable for color printing. Since the display screen of a personal computer displays a color image, the ink jet printing method is very suitable as a color printer for a personal computer and is already in practical use.

As described above, the ink jet printing method has a problem of ink bleeding, and when the ink bleeds, the contour of the image is blurred, so that the printed matter looks remarkably inferior in quality. In plain paper with pulp fibers exposed on the surface, ink jet printing cannot be performed on plain paper because the ink easily bleeds due to capillary action. Therefore, in the current ink jet printer, bleeding is prevented by using a special paper that does not bleed ink. The dedicated paper has an ink absorbing layer coated on the paper, and silica fine powder having a large liquid absorbing ability is applied as an ink absorbing layer together with an adhesive. However, since dedicated paper is expensive and not suitable for paper recycling, there is a strong demand for an ink jet printing method that does not bleed even when plain paper is used.

This problem has been solved by the method described in the specification of Japanese Patent Application No. 9-154246 or Japanese Patent Application No. 2000-50312. That is, it is an ink jet printing method in which the water-soluble dye contained in the ink is converted into an insoluble dye by UV irradiation or laser irradiation and is fixed, thereby preventing ink bleeding. With this method, ink jet printing can be performed even on plain paper while preventing ink bleeding. At the same time, since the water resistance of the ink dye is improved, the ink jet printing method can be applied to plain paper without any problems, and it has become possible to develop as a practical printing method.

[0008]

However, in the ink jet printing method of Japanese Patent Application No. 9-154246 or Japanese Patent Application No. 2000-50312, only the insolubilized dye is fixed on the paper surface. For example, OH
The ink dye printed on the P transparent plastic sheet has a problem of poor abrasion resistance. That is, there is a problem in that the ink pigment is removed when the printed surface is rubbed with a fingertip. This problem is due to the fact that no binder can be added to the ink for ink jet. In addition, a printed image of a color image often requires a glossy printing surface.
Since a printed matter having gloss can obtain a high-grade feeling, an ink for ink jet printing which gives gloss has been demanded.
For example, when printing a color image taken with a digital camera, if a glossy image like a photographic printing paper of a film type color photograph is obtained, it becomes a very high-quality printed matter, and a high-quality image image is obtained. give.
Since the ink for ink jet printing is provided with abrasion resistance and gloss, there has been a problem of how a binder can be added to an ink composition without increasing the viscosity of the ink.

[0009]

As described above, in the ink jet printing method described in Japanese Patent Application No. 9-154246 or Japanese Patent Application No. 2000-50312, UV is applied to the ink on the paper.
It is a method of irradiating light or laser to fix the ink. At this time, by adding a photopolymerizable monomer to the ink composition, UV for fixing the ink
Irradiation has been found to give glossy ink jet prints. In this method, the ink coloring material is firmly adhered to the material to be printed without increasing the viscosity of the ink, and the printed matter has abrasion resistance and gloss.

As the photopolymerizable monomer, water-soluble monomethacrylate, monoacrylate, dimethacrylate,
Diacrylate or acrylamide can be used. The photopolymerizable monomers listed in Table 1 below have good solubility in water, low viscosity, and low skin irritation, and are suitable for addition to the ink composition of the present invention.

[0011]

[Table 1]

As described above, the ink for ink jet printing is basically an ink composition composed of water and a water-soluble dye. However, in practice, a small amount of a non-volatile hydrophilic solvent is added in order to prevent clogging of the nozzle due to evaporation of the ink solvent in the nozzle opening.
Diethylene glycol or N as a non-volatile hydrophilic solvent
-Methyl-imidazolidinone and the like are used. Instead of these non-volatile hydrophilic solvents, the water-soluble photopolymerizable monomers listed above can be added to the ink.
A dimer (dimer) may be used in combination with these photopolymerizable monomers. Increasing the mixing ratio of dimers increases the hardness of the printed ink. A small amount of a polymerization inhibitor, such as handloquinone or metoquinone, may be added to these photopolymerizable monomers.

In the usual photopolymerization reaction, a photoinitiator (= photoreaction initiator) is required when the polymerizable monomer is photopolymerized by UV irradiation. The photoinitiator is activated by UV to generate radicals to start the polymerization reaction. Photoinitiators include benzoin, benzophenone, benzyl,
Aromatic ketone compounds such as Michler's ketone (4,4′-bisdimethylaminobenzophenone) and benzyldimethylketal are used. The chemical structural formulas of these aromatic ketone compounds are shown in the following chemical formula 1.

[0014]

[Chemical 1]

The aromatic ketone compound shown in Chemical formula 1 functions as a photoinitiator because it absorbs UV to form a radical by molecular cleavage or hydrogen abstraction. For example, the following chemical formula 2 shows how benzoin and benzophenone are activated by UV to generate radicals. There is also a method of enhancing the effect of the photoinitiator by using a sensitizer (tertiary amine) together.

[0016]

[Chemical 2]

In order to photopolymerize the polymerizable monomer, a photoinitiator which absorbs UV and generates a radical is required.
The present inventor has discovered that in the ink composition of the present invention, a soluble vat dye functions to initiate a polymerization reaction. This is an interesting phenomenon newly discovered this time, and the present invention was embodied based on this new finding. The aromatic ketone compound as the photoinitiator listed in Chemical formula 1 is insoluble in water, and has a limited storage stability, so that it cannot be added to an aqueous ink.
On the other hand, it was revealed that the soluble vat dye is effective as an initiator of radical polymerization reaction because it is water-soluble and is added as a coloring material in a large amount to the ink.
The soluble vat dye (sulfate ester salt of the leuco compound of the dye) contained in the ink of the present invention is very sensitive to UV light and easily activated. It is considered that the soluble vat dye absorbs UV to generate a leuco body, and the excited active radicals of the leuco body initiate a polymerization reaction. The following chemical formula 3 shows a photochemical reaction process when a soluble vat dye absorbs UV to generate a quinone dye (aromatic ketone compound).

[0018]

[Chemical 3]

The active radical generated by being excited by UV irradiation or laser irradiation is added to the double bond of the polymerizable monomer to initiate the radical polymerization reaction. It was revealed that the soluble vat dye is a strong initiator of radical polymerization reaction. It is considered that since the molecule is large, it has a large ability to absorb UV energy, and as a result of being excited to a high potential energy state, a radical that is strongly activated is generated to start the photopolymerization reaction. The present invention was reached only after the discovery that a soluble vat dye initiates a radical polymerization reaction by UV irradiation or laser irradiation. When the aqueous solution of the soluble vat dye is irradiated with UV, the soluble vat dye is hydrolyzed, but if a polymerizable monomer that is more easily reactive than water is present, a radical polymerization reaction proceeds prior to the hydrolysis. Therefore, when UV irradiation is applied to an aqueous ink containing a soluble vat dye and a polymerizable monomer, the ink can be fixed more quickly and the ink can be fixed without bleeding as compared with the aqueous ink containing only the soluble vat dye. It became possible. Ink jet printing of high-quality images having gloss on plain paper is possible.

The soluble vat dye is a water-soluble dye obtained by reducing a vat dye (= but dye) to obtain a stable compound by esterifying the unstable --OH group of a leuco body with sulfuric acid. . When the carbonyl group (> C = O) of vat dye is reduced, carbino It is unstable because it is bonded, and it has a property that hydrogen is easily released to return to the original carbonyl group (vat dye). Therefore, soluble vat dyes are products that are easy to use by esterifying them into stable compounds. When the unstable —OH group of the leuco body is esterified with a dibasic acid, sulfuric acid,
Soluble vat dyes are those in which one acid group forms an ester with OH and the other acid group is converted to a sodium salt and left as a water-soluble group.

Normal pigment compounds and dyes have absorption in the visible wavelength range. Soluble vat dyes are exceptionally ultraviolet wavelength region 2
The reason for having absorption at 50 to 400 nm is explained as follows. The dye compound of soluble vat dye is a reduced form of butt dye, and when it is reduced, the number of double bonds in the conjugated double bond system of the dye compound decreases by one, so that the absorption spectrum of the dye compound has a short wavelength. Move to the side. Therefore, in the case of a soluble vat dye having a small molecular weight such as indigo, the absorption spectrum may completely shift to the ultraviolet wavelength range and become colorless. The ink jet printing method of the present invention is a method of utilizing UV absorption of the soluble vat dye to generate an unstable leuco body and initiating a polymerization reaction by the active radicals of the excited leuco body.

According to the ink jet printing method of the present invention, not only a glossy printed matter can be obtained, but also ink fixing without ink bleeding on plain paper or recycled paper is possible. It is considered that this is because the ink is instantly gelled or cured by UV irradiation or laser irradiation. Since the ink composition of the present invention can prevent the ink solvent from bleeding, the ink dots can be fixed completely on the plain paper without the ink bleeding, and the ink jet printing can be performed at a high speed. Moreover, since the ink dots have gloss, and the ink color material is a pigment of ultrafine particles converted from a water-soluble dye, a clear transparent color can be obtained, and a high-quality color image can be obtained. According to the ink jet printing method of the present invention, ink jet printing can be performed even on a transparent plastic sheet for OHP. In addition to paper as printing materials, weaving, knitting, non-woven fabric, wood, plastic, synthetic leather, natural leather, glass, ceramics,
Inkjet printing can be performed on a material such as metal, and a printed matter having excellent durability (rubbing resistance, heat resistance, light resistance, water resistance, oil resistance) can be obtained.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The ink used in the ink jet printing method of the present invention has the following composition: 10 to 80 parts by weight water-soluble photopolymerizable monomer 1 to 10 parts by weight soluble vat dye 80 to 10 parts by weight water 100 The water-soluble polymerizable monomer is selected from those listed in Table 1. The larger the amount of the polymerizable monomer added, the stronger the gloss. Also, the higher the mixing ratio of dimers, the harder the printed surface. The addition amount of the polymerization inhibitor is preferably large considering the storage stability of the ink, and is adjusted in the range of 1 to 100 mg / 1. Due to the high content of soluble vat dyes, the polymerization reaction does not slow down. Since the soluble vat dye remains as a coloring material after being used as a polymerization initiator, a color tone suitable for each of color ink and black ink is selected.
The soluble vat dyes suitable for 7 color inks of CMYRGBK (cyan, magenta, yellow, red, green, blue, black) are shown in Chemical formula 4. 7 color ink
As described in Japanese Patent Application No. 2000-86433, the ink system has improved fidelity of color reproduction in color printing.

[0024]

[Chemical 4]

After ink jet printing using the ink having the above composition, the printing surface is irradiated with UV. When ultraviolet rays are irradiated by a UV lamp, it is effective to use a low pressure mercury lamp and an ultraviolet fluorescent lamp (or a metal halide lamp) described in JP-A-10-305570 together. Further, in the case of laser irradiation, an ultraviolet laser is effective, and for example, a gallium nitride semiconductor laser having an output wavelength of 403 nm as described in Japanese Patent Application No. 2000-50312 is suitable.

[0026]

Example 1 In order to prepare color inks of CMY (cyan, magenta, yellow) three primary colors, an aqueous ink having the following composition was prepared. 25 parts by weight methoxydiethylene glycol methacrylate 30 parts by weight N, N-dimethylacrylamide 5 parts by weight Soluble vat dye *) 40 parts by weight Water 100 *) The soluble vat dyes used were the dyes for the following three CMY primary colors. Ink jet printing was performed on plain paper using the above CMY primary color inks and laser irradiation was performed to obtain a glossy color image. A gallium nitride semiconductor laser device having a wavelength of 403 nm was used for the laser.

[0027]

Example 2 To make a black ink, an aqueous ink having the following composition was prepared. 30 parts by weight Methoxydiethylene glycol methacrylate 30 parts by weight N, N-dimethylacrylamide 8 parts by weight Soluble vat dye *) 32 parts by weight Water 100 *) The following black-colored compounded dye was used as the soluble vat dye. Using the above black ink, ink jet printing was performed on a transparent plastic sheet for OHP, and UV irradiation was performed to obtain a scratch-resistant printed matter. Even if the surface of the printed matter was rubbed with a finger, the ink did not come off. A 15 W low-pressure mercury lamp and a 20 W ultraviolet fluorescent lamp were used together for UV irradiation.

[0026]

As described above, after the ink jet printing is performed using the water-based ink containing the polymerizable monomer and the soluble vat dye, the ink on the printed surface is irradiated with UV or laser to produce a plain paper or a recycled paper. Even on the rough paper, a color image having no ink bleeding and having gloss can be obtained. The ink jet method has good compatibility with a computer and is suitable as a means for printing a digital image signal on paper, and is superior to an electrophotographic method in printing a color image. Conventionally, the ink jet printing method has been used only as a small personal printer because highly practical ink has not been provided, but by using the ink of the present invention, the ink jet method can be used also in the commercial printing field. It was Since the problem of ink bleeding has been solved, it has become easier to increase the printing speed. Since the ink jet ink has a simple composition, it has an advantage that a printer can prepare the ink by himself. In commercial printing, low ink costs are important. Since the ink formulation only dissolves the dye in water, the dye is supplied in powder or tablet form. Since the polymerizable monomer is used as the ink solvent, the printing head is cleaned with water after the printing operation of one day is completed. In this respect, the same applies to the conventional plate printing, and the printing rolls and blankets are cleaned when the printing work of one day is completed. Since oil-based ink is used in plate printing, it is washed with an organic solvent.
Air pollution due to solvent vapor becomes a problem. Since the ink jet printing method uses a water-based ink, it only needs to be cleaned with water, and an ink system that supplies only a dye is an environmentally friendly printing method.

Claims (3)

[Claims] 1. An ink jet printing using an ink containing a polymerizable monomer and a soluble vat dye as a main solvent of water, and then applying light energy to the ink on a printing material to fix the ink. Ink Jet printing method. 2. The ink jet printing method according to claim 1, wherein the polymerizable monomer is methoxypolyethylene glycol methacrylate and / or N, N-dimethylacrylamide represented by the following general formula. 3. The ink jet printing method according to claim 1, wherein the light energy is a laser.