JP7507631B2 - Charge-controlled ink for inkjet printers - Google Patents

Description

The present invention relates to ink for charge-controlled inkjet printers.

Since charge-control inkjet printers are capable of printing on products with three-dimensional shapes, they are used in a wide range of fields, including food and electronic components, to print best-before dates, use-by dates, serial numbers, and other information.

Ink for charge-controlled inkjet printers generally contains resin, colorant, solvent, and conductive agent. Additives such as leveling agents may also be added to control the shape of the printed dots.

Patent document 1 describes how adding a fluorescent pigment to an aqueous pigment-based ink can improve the saturation of the ink.

Japanese Patent Application Laid-Open No. 10-130558

Many electronic components are black or brown in color, such as brown, tan, or gray, so it is necessary to ensure sufficient visibility even when printing on black or brown colored components.

Patent Document 1 does not take into consideration printing on black or brown substrates such as electronic components. Furthermore, depending on the type of fluorescent pigment, adding it to the ink may result in the print showing the same color as the substrate, and the visibility of the print may not improve.

Therefore, the present invention aims to provide an ink that can produce highly visible prints when printed on black or brown substrates such as electronic components.

To achieve the above object, the ink for charge-controlled inkjet printers according to the present invention is characterized by containing a conductive agent, an organic solvent, a resin, a pigment containing carbon black, and a fluorescent material having a maximum emission wavelength in the wavelength range of 400 nm or more and 500 nm or less.

The present invention is capable of supplying ink that can produce highly visible prints when printed on black or brown substrates such as electronic components.

Problems, configurations, and advantages other than those described above will become clear from the description of the embodiments below.

A diagram showing the relationship between human visual sensitivity and wavelength Reflected light intensity of printed dots formed with ink containing no fluorescent material and emission intensity spectrum of compound 1 Light intensity spectrum of printed dots formed with ink containing compound 1 Emission intensity and absorbance spectrum of compound 2 Reflected light intensity spectra of dots printed with ink containing or not containing compound 2 Schematic diagram of the printing process of a charge control type inkjet printer Emission intensity and absorbance spectrum of compound 3 Prints formed using the ink of Example 1 and the ink of Comparative Example 1 Reflected light intensity spectra of dots printed with ink containing or not containing compound 3 Emission intensity and absorbance spectrum of compound 4 Reflected light intensity spectra of dots printed with ink containing or not containing compound 4 Emission intensity and absorbance spectrum of compound 5 Reflected light intensity spectra of dots printed with ink containing or not containing compound 5 Emission intensity and absorbance spectrum of compound 6 Reflected light intensity spectra of dots printed with ink containing or not containing compound 6

The following describes an embodiment of the present invention with reference to the drawings.

<Ink>
The ink contains a resin, a colorant, an organic solvent, a conductive agent, and may contain other additives, dispersants, etc. These are stirred with a stirrer tip or an overhead stirrer, etc., to make them compatible with each other and form the ink.

<Resin>
The resin mainly serves as a support for the print. In other words, it serves to support colorants such as dyes and pigments, and other additives, in the print dots. It also serves to ensure abrasion resistance, adhesion, and other properties that are acceptable for practical use. In addition, the resin used can be one that dissolves in the ink solvent.

To improve visibility, it is preferable to use a resin that does not have a benzene ring. This is because if a resin has a benzene ring, when the light irradiated is 350 nm or less, the resin itself may absorb part of the light, which may reduce the amount of light emitted.

The ink according to one embodiment of the present invention can be used to print serial numbers and the like on electronic components. The ink used for printing on electronic components must withstand the temperature required to melt the solder balls for soldering after printing. The printed dots are heated at approximately 240°C to 260°C for 2 to 3 minutes. The substrate to which the solder is to be joined is coated with a flux that improves the wettability of the solder, but after soldering, the flux is removed by washing with ethanol or a low-boiling alcohol such as 2-propanol, which has a boiling point of approximately 100°C or less, so it is also desirable for the ink to be poorly soluble in alcohol. Therefore, it is preferable that the ink to be printed on electronic components withstands solder reflow at 240°C to 260°C after printing and does not dissolve in alcohol such as ethanol or 2-propanol used to remove the flux thereafter.

As a result of our investigation, we found that dots printed with ink that uses a resin with a hydrocarbon main chain, specifically acrylic resin or styrene-acrylic resin, swell slightly when immersed in ethanol or 2-propanol, and gradually peel off.

In contrast, inks using polyesters whose main chains are formed by condensation of dicarboxylic acids and diols did not peel off even when immersed in alcohol. In particular, when the dicarboxylic acid forming the polyester has a benzene ring, the ink containing this polyester was found to have high alcohol resistance. Specific examples of dicarboxylic acids having a benzene ring include phthalic acid, isophthalic acid, and terephthalic acid. In addition, when the diol forming the polyester has a benzene ring, the ink containing this polyester was found to have high alcohol resistance. Specific examples of diols having a benzene ring include benzyl alcohol. It is presumed that the reason for the high alcohol resistance is that the benzene rings of multiple molecular chains stack together, narrowing the gap between the molecular chains and suppressing the intrusion of alcohol between the molecular chains.

However, if all the dicarboxylic acids and diols constituting the polyester are made of materials having a benzene ring, the polyester will not dissolve in the ketone-based solvents described below. Therefore, it is preferable to use a polyester resin that also contains a certain amount of aliphatic dicarboxylic acids or aliphatic diols. In other words, it is preferable to use a polyester resin that contains a dicarboxylic acid or diol having a benzene ring and an aliphatic dicarboxylic acid or aliphatic diol.

Polyester resins are synthesized by ester bonds between dicarboxylic acids and diols, and have carboxyl or hydroxyl groups at the terminals. These substituents are hydrophilic, and if their proportion is high, they tend to swell easily in alcohol. To ensure sufficient alcohol resistance, it is desirable to reduce the number of these substituents. To improve alcohol resistance, it is desirable to set the total of the acid value, which indicates the proportion of these substituents, and the hydroxyl value to 60 mg (KOH)/g or less. For example, by increasing the degree of polymerization and increasing the molecular weight, the proportion of these substituents can be reduced, and the total of the acid value and hydroxyl value can be set to 60 mg (KOH)/g or less.

<Coloring Agent>
Carbon black is used as the colorant instead of a black dye. Printing on electronic components is heated to 240° C. or higher and 260° C. or lower during solder reflow, so if a dye is used, the dye may denature and change to brown, reddish brown, or the like. Therefore, visibility is reduced when the substrate is brown or red. Therefore, the ink according to one embodiment of the present invention uses carbon black, which does not change color under the heating conditions during solder reflow.

The true specific gravity of carbon black is 1.8 to 1.9, which is larger than that of ketone-based solvents, which have a specific gravity of 0.8 to 0.9. Therefore, in order to suppress precipitation in the ink, it is preferable that the average particle size of carbon black is less than 1 μm. Furthermore, the smaller the particle size of carbon black, the larger the surface area per unit weight, so that a small addition rate makes the blackness visible. From the viewpoint of visibility, it is even more preferable that the average particle size of carbon black is 0.5 μm or less.

On the other hand, the smaller the average particle size, the more likely it is that the particles will float in the air when weighing the ink when preparing it, when pouring it into a container used for preparation, etc. Also, static electricity makes the particles more likely to adhere to various parts of the preparation area. Therefore, when considering ease of handling, it is desirable for the average particle size to be 20 nm or more.

<Dispersant>
When carbon black aggregates into large lumps, it is prone to settling and may cause problems such as clogging the ink piping and nozzles of ink-jet printers. In order to suppress this carbon black aggregation, a dispersant may be added. The dispersant adheres to the periphery of the carbon black particles and improves their dispersibility in the solvent.

In order to increase the compatibility between the dispersant and the resin, it is preferable that the dispersant has an ester structure in its molecular structure. It is also preferable that the dispersant has a cyclic structure. A dispersant having a cyclic structure exerts dispersibility by adhering to a single carbon black particle, and therefore can suppress the aggregation of carbon black particles. Specific examples of esters having a cyclic structure that can be used include β-propiolactone, γ-butyrolactone, γ-caprolactone, γ-nonalactone, γ-decalactone, γ-undecalactone, 3-methyloctano-4-lactone, δ-valerolactone, ε-caprolactone, cyclopentadecanolide, and cyclohexadecanolide.

The above dispersants are liquid at room temperature, and if they remain inside the printed dots for a long time, problems such as a decrease in the abrasion resistance of the print will occur. Therefore, like a solvent, it is preferable for them to disappear from inside the printed dots by volatilizing after printing. Therefore, it is preferable for the dispersant to have a boiling point of 300°C or less at normal pressure. Specific examples include β-propiolactone, γ-butyrolactone, γ-caprolactone, γ-nonalactone, γ-decalactone, γ-undecalactone, δ-valerolactone, ε-caprolactone, cyclopentadecanolide, and cyclohexadecanolide.

In addition, polycaprolactone, a polymer of caprolactone, is also cross-linked by ester bonds and is suitable as a dispersant. The lower the number average molecular weight, the less it affects the viscosity of the ink, so it is preferable. Specifically, a number average molecular weight of approximately 25,000 or more and 100,000 or less is suitable. As mentioned above, if the number average molecular weight is larger than this, the viscosity of the ink tends to increase. This makes it necessary to lower the concentration of the resin. However, lowering the concentration of the resin may cause a decrease in abrasion resistance and adhesion. Therefore, from the perspective of suppressing the decrease in abrasion resistance and adhesion, the number average molecular weight needs to be kept at a maximum of approximately 100,000.

In order to prevent pigment aggregation, it is preferable that the amount of dispersant added is 25 wt% or more and 75 wt% or less relative to the pigment. In order to prevent a decrease in the physical strength of the print, it is even more preferable that the amount of dispersant added is 25 wt% or more and 50 wt% or less relative to the pigment.

<Fluorescent materials>
The fluorescent material may be a material having a maximum emission wavelength in the wavelength range of 400 nm or more and 500 nm or less. The fluorescent material may be a fluorescent dye dissolved in the ink solvent or a fluorescent pigment dispersed in the ink. Comparing dyes and pigments, it is preferable to use dyes that do not cause a decrease in reflected light intensity due to Rayleigh scattering.

The effect of adding a fluorescent material is explained below. The ink according to one embodiment of the present invention is a black pigment ink with dispersed carbon black. Usually, the visibility of printed dots using pigment ink is reduced, especially on a brown substrate. If the printed dots are carefully observed, they will appear brown, even though they should be black. Therefore, they will be similar in color to the substrate, and it is believed that this reduces visibility. The human eye sees the reflected light of the printed dots. Figure 1 shows the relationship between human luminosity and wavelength. As shown in Figure 1, the visible range for humans is 400 nm to 700 nm. The reason why the printed dots appear brown is believed to be that the reflected light intensity of purple and blue light in the range of 400 nm to 500 nm is reduced compared to the reflected light intensity of yellow, orange, and red light in the range of 570 nm to 700 nm.

The reason for the decrease in reflected light intensity of purple and blue light in the range of 400 nm to 500 nm will be explained. Figure 2 shows the reflected light intensity 1 of a printed dot formed with a pigment ink to which no fluorescent material has been added, and the emission intensity spectrum 2 of 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (hereinafter referred to as compound 1), a fluorescent material with a maximum emission wavelength in the wavelength range of 400 nm to 500 nm. As shown in Figure 2, it was found that the reflected light intensity 1 of the printed dot decreases as the wavelength becomes shorter due to Rayleigh scattering caused by the pigment fine particles in the printed dot. In other words, it is believed that the cause of the decrease in visibility of the printed dot on the surface of the brown substrate is Rayleigh scattering caused by the pigment fine particles.

As can be seen from Figure 1, visibility is at its maximum when the wavelength is around 550 nm. Therefore, in order to improve visibility, it is necessary to increase the emission intensity in the blue to purple region, which is a short wavelength region in the visible light range. In other words, it is considered preferable to add a fluorescent material that has a maximum emission wavelength in the wavelength range of 400 nm to 500 nm.

Figure 3 shows the light intensity spectrum formed by the pigment ink to which compound 1 has been added. As shown in Figure 3, when compound 1 is added to the pigment ink, the reflected light intensity 3 of the print dot increases in the short wavelength region of the visible range from 400 nm to 500 nm, improving visibility. In addition, at the concentration of compound 1 contained in the print dot, the absorption coefficient in the visible range of 400 nm or more is below the measurement limit of the ultraviolet-visible spectrometer. In other words, it is recognized as colorless by the naked eye. The fluorescent material is added to compensate for the reflected light intensity of 400 nm to 500 nm or less that is reduced by Rayleigh scattering. Therefore, if a fluorescent material that does not have a maximum emission wavelength in the wavelength range of 400 nm to 500 nm or less is used, visibility will not be improved.

Figure 4 shows the emission spectrum 4 of compound 2 (a red phosphor made by Sialon Co., Ltd.), a fluorescent material with an emission range of 500 to 840 nm with a maximum value at 655 nm, and the absorbance spectrum 5 at the concentration contained in the printed dot. As shown in Figure 4, compound 2 has an emission range of 500 to 840 nm, so it does not have the effect of compensating for reflected light from 400 to 500 nm, where the reflected light intensity is significantly reduced due to Rayleigh scattering. On the contrary, it increases the intensity of yellow, orange, and red from 570 to 700 nm, making the print appear more brown. Therefore, the visibility of printed dots on a brown substrate is reduced compared to when compound 2 is not added. Furthermore, looking at the absorbance 5 of compound 2, it can be seen that it absorbs light from 350 to 650 nm. In particular, the absorption rate is higher in the short wavelength range of 500 nm or less. Therefore, the reflected light intensity from 400 to 500 nm is further reduced.

Figure 5 shows the reflected light intensity 6 of a printed dot formed with a pigment ink containing compound 2, and the reflected light intensity 7 of a printed dot formed with a pigment ink containing no fluorescent material such as compound 2. It was found that by adding compound 2, the reflected light intensity in the long wavelength region of the visible range from yellow to red becomes stronger, and conversely, the reflected light intensity in the short wavelength region from purple to blue becomes weaker. In other words, the addition of compound 2, a fluorescent material, makes the print appear browner, and therefore the visibility of the printed dot on a brown substrate is greatly reduced.

From the above, it can be said that it is preferable to add a fluorescent material with an emission maximum at wavelengths between 400 nm and 500 nm to the black pigment ink. In addition, it is preferable that the fluorescent material emits as little yellow, orange, and red light at wavelengths above 570 nm as possible. Specifically, it is preferable that the emission intensity at 570 nm is 30% or less of the emission intensity at the maximum emission wavelength. In addition, it is preferable that the absorbance of the fluorescent material in the ink in the wavelength range between 400 nm and 500 nm is smaller than the emission intensity so that the reflection intensity is not reduced due to absorption by the fluorescent material.

Fluorescent materials with a maximum emission wavelength in the wavelength range of 400 nm to 500 nm include, for example, 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole), 7-diethylamino-4-methylcoumarin, and 4,4'-bis(2-sulfostyryl)-biphenyl disodium salt. In particular, 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole), which has high luminous efficiency, is suitable.

The higher the content of fluorescent material in the ink, the greater the amount of light emitted, so it is necessary to add at least 0.1 wt% to the ink. However, if too much is added, the proportion of resin added to increase the physical strength of the print decreases, and the strength of the print tends to decrease. For this reason, it is preferable to add no more than 1 wt%. Therefore, it is preferable to add 0.1 wt% to 1 wt% of the ink. From the perspective of improving the strength of the print when printed on electronic components, it is even more preferable to add 0.3 wt% to 0.5 wt%.

<Organic Solvent>
As the organic solvent, a solvent having a boiling point of 70° C. or more and 140° C. or less and drying quickly after printing can be used. In addition, as the organic solvent, it is preferable to use a solvent that is not infinitely diluted with water. In other words, it is preferable to use a solvent that is partially incompatible when mixed with water. By using a solvent that is not infinitely diluted with water, it is possible to suppress changes in the ink characteristics caused by the dissolution of moisture in the air into the solvent. Specific examples of the solvent that is not infinitely diluted with water include 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 2-octanol, 2-butanone, 3-methyl-2-butanone, 2-pentanone, 3-pentanone, pinacolone, cyclohexanone, diethyl ether, dipropyl ether, dibutyl ether, ethyl acetate, propyl acetate, butyl acetate, ethyl propionate, propyl propionate, butyl propionate, and the like. Specific examples of solvents that are infinitely dilutable with water include methanol, ethanol, 1-propanol, 2-propanol, isobutanol, tert-butanol, ethylene glycol, diethylene glycol, triethylene glycol, glycerin, acetone, tetrahydrofuran, 1,3-dioxolane, and 1,4-dioxane.

Among solvents that are not infinitely diluted with water, the main component of the solvent contained in the ink is preferably a ketone solvent such as 2-butanone, 3-methyl-2-butanone, 2-pentanone, 3-pentanone, pinacolone, or cyclohexanone. In this specification, the main component refers to the component that is contained in the largest amount. In order to reduce the drying time of the printed dots after printing, it is more preferable to use acetone, MEK, 2-pentanone, 3-pentanone, 3-methyl-2-butanone, or a mixture of these. Printing is possible using solvents with a higher carbon number than these, but the drying time of the printed dots after printing will be longer.

<Additives>
Additives contained in the ink according to one embodiment of the present invention include a conductive agent that imparts electrical conductivity to the ink, and a leveling agent that forms flat printed dots.

Ink used in charge control type inkjet printers is required to be conductive. For this reason, a conductive agent is added. Examples of conductive agents include those with a metal salt structure. However, the conductive agent must be soluble in a solvent such as 2-pentanone, 3-pentanone, or 3-methyl-2-butanone, and is preferably one that does not corrode, dissolve, or swell the pump and hose components inside the inkjet printer. In consideration of these factors, tetraalkylammonium tetraphenylborate, tetraalkylammonium hexafluorophosphate, tetramethylammonium tetraphenylborate, pyridinium salts with alkyl chains, and the like are preferred. As long as the required conductivity can be ensured, there are no particular restrictions on the amount of conductive agent added.

Leveling agents have the effect of flattening printed dots or preventing the dots from spreading. This makes it possible to form flat, small dots. These effects are hereinafter described in the specification as leveling properties. Generally, compounds having a polydimethylsiloxane chain and having a polyalkoxy group at the end or side chain of this chain, or compounds having an alkylamino group, are common. In addition, compounds in which a silicon-containing group is crosslinked to a styrene-methacrylate chain are known. In the case of the ink of the present invention, there are no particular restrictions as long as it is soluble in the solvent used.

<Inkjet printer>
The ink described above can be printed in a charge control type ink jet printer.

Figure 6 shows the process of ink ejection and landing in an inkjet printer using the charge control method.

The ink droplets 9 ejected from the nozzle 8 are given an electric charge by the charging electrode 10. The direction is then controlled by the deflection electrode 11, and the droplets land on the print target 12. Although not shown in Figure 6, this inkjet printer also has a mechanism for heating the nozzle 8. This plays the role of heating the ink when it is highly viscous, thereby reducing the viscosity to the desired level by the inkjet printer.

Although not shown in Figure 6, the ink that is not printed is collected through the gutter 13 and returned to the ink tank.

The ink according to one embodiment of the present invention will be explained in more detail below through various experiments.

816 g of 2-butanone was added to a 2 L SUS304 container. The mixture was stirred using an overhead stirrer at a speed that did not cause the 2-butanone to splash. During stirring, 130 g of polyester resin A with a weight average molecular weight of 12,000 and a glass transition temperature of approximately 50°C, 36 g of carbon black with an average particle size of 50 nm, 2 g of δ-valerolactone as a dispersant, 3 g of compound 3 (a broadband blue phosphor manufactured by Sialon Co., Ltd.) as a fluorescent dye, 1 g of a polydimethylsiloxane chain compound having polyalkoxy groups at both ends as a leveling agent, and 12 g of tetramethylammonium hexafluoroborate as a conductive agent were added. In this way, the ink was prepared. Figure 7 shows the emission spectrum and absorption spectrum of compound 3.

An ink was prepared in the same manner as in Example 1, except that 3 g of compound 4 (a blue phosphor manufactured by Sialon Corporation) was used as the fluorescent dye instead of 3 g of compound 3.

An ink was prepared in the same manner as in Example 1, except that 3 g of compound 5 (narrow linewidth blue phosphor manufactured by Sialon Corporation) was used as the fluorescent dye instead of 3 g of compound 3.

An ink was prepared in the same manner as in Example 1, except that 100 g of polyester resin B, which has a weight average molecular weight of 22,000 and a glass transition temperature of approximately 30°C, was used instead of polyester resin A, and the amount of 2-butanone added was 846 g.

An ink was prepared in the same manner as in Example 1, except that 160 g of polyester resin D, which has a weight average molecular weight of 6,000 and a glass transition temperature of approximately 60°C, was used instead of polyester resin A, and the amount of 2-butanone added was 846 g.

(Comparative Example 1)
An ink was prepared in the same manner as in Example 1, except that no fluorescent material was added and the amount of 2-butanone added was 819 g.

(Comparative Example 2)
An ink was prepared in the same manner as in Example 1, except that 3 g of Compound 6 (a broadband blue-green phosphor manufactured by Sialon Corporation) was used as the fluorescent dye instead of 3 g of Compound 3.

An ink was prepared in the same manner as in Example 1, except that 110 g of polyester resin C, which has a weight average molecular weight of 18,000 and a glass transition temperature of approximately 0°C, was used instead of polyester resin A, and the amount of 2-butanone added was 836 g.

An ink was prepared in the same manner as in Example 1, except that 190 g of polyester resin E, which has a weight average molecular weight of 4,000 and a glass transition temperature of approximately 60°C, was used instead of polyester resin A, and the amount of 2-butanone added was 876 g.

An ink was prepared in the same manner as in Example 1, except that 160 g of acrylic resin F, which has a weight-average molecular weight of 10,000 and a glass transition temperature of approximately 74°C, was used instead of polyester resin A, and the amount of 2-butanone added was 846 g.

An ink was prepared in the same manner as in Example 1, except that 160 g of styrene/acrylic resin G, which has a weight average molecular weight of 8,000 and a glass transition temperature of approximately 65°C, was used instead of polyester resin A, and the amount of 2-butanone added was 846 g.

Table 1 shows the monomer units, glass transition temperature, number average molecular weight, acid value, hydroxyl value, etc. that make up each resin for the polyester resins A to E, acrylic resin F, and styrene/acrylic resin G used in each example and comparative example.

The viscosity of the ink changes depending on the type and amount of resin added. Other additives do not have as great an effect as the resin. Therefore, the amount of resin added to the ink according to one embodiment of the present invention is adjusted so that the viscosity of the ink prepared is in the range of 3.0 mPa·s to 3.5 mPa·s at 20°C. When the type of resin is the same, as in Examples 1 to 3, the amount added is not changed.

(2) Printing The inks prepared in Examples 1 to 9 and Comparative Examples 1 and 2 were filled into the ink tank of an inkjet printer PX-R manufactured by Hitachi Industrial Equipment Co., Ltd. After filling, printing was performed on an inductor having a printing surface size of 4×4 mm and a thickness of 1.5 mm, and the printing surface was visually observed to be brown.

(3-1) Visibility Evaluation Method Whether or not the contents indicated by the printed dots were visible was evaluated from a position 30 cm away from the prints formed with the inks of Examples 1 to 9 and Comparative Examples 1 and 2. If it was visible, it was rated as "OK", and if it was difficult to see, it was rated as "No".

(3-2) Method for Evaluating Heat Resistance Heat resistance was evaluated by printing the inks according to Examples 1 to 9 and the inks according to Comparative Examples 1 and 2 on an inductor, leaving the inductor with the printing in a thermostatic chamber at 240° C. for 3 minutes, and then observing the printed dots. After that, the inductor was left in a thermostatic chamber at 260° C. for 3 minutes, and the printed dots were observed. If the shape of the printed dots changed after being left in the high-temperature chamber at 240° C. for 3 minutes, it was judged as "changed", and if the shape of the printed dots did not change even after being left in the high-temperature chamber at 260° C. for 3 minutes, it was judged as "no change".

(3-3) Method for Evaluating Ethanol Resistance Ethanol resistance was evaluated by leaving the inductors printed with the inks according to Examples 1 to 9 and the inks according to Comparative Examples 1 and 2 in a petri dish containing ethanol at 40° C. for 20 minutes, and then visually observing whether the printed ink changed before and after immersion in ethanol. When no change in the print was observed by visual observation even after immersion in ethanol at 40° C., it was rated as "visually confirmed". When it was confirmed that the print had become faint after immersion in ethanol at 40° C., it was rated as "faint". When it was confirmed that the print had disappeared after immersion in ethanol at 40° C., it was rated as "disappeared".

The results for the visibility, heat resistance, and ethanol resistance of the printed dots or prints formed with the inks of Examples 1 to 9 and Comparative Examples 1 and 2 are shown in Table 2.

The visibility evaluation, heat resistance evaluation, and ethanol resistance evaluation are explained in detail below.

(4-1) Visibility Evaluation Results Fig. 8 shows prints formed with the ink of Example 1 and the ink of Comparative Example 1. The print formed with the ink of Comparative Example 1 had low visibility, and the contents indicated by the printed dots were unclear when visually observed from a distance of 30 cm. However, the print formed with the ink of Example 1 had high visibility, and the contents indicated by the print could be confirmed even from a distance of 30 cm.

As shown in Table 2, the prints formed with the inks of Examples 2 to 9, like the prints formed with the ink of Example 1, had high visibility, and the contents of the prints could be confirmed from a distance of 30 cm. However, the prints formed with the ink of Comparative Example 2, like the print dots formed with the ink of Comparative Example 1, had low visibility, and the contents of the prints could not be confirmed from a distance of 30 cm.

The reason why the visibility of the prints in Examples 1, 4 to 9, in which Compound 3 was added as a fluorescent material, was good is as follows. Figure 7 shows the emission spectrum 14 of Compound 3 and the absorbance spectrum 15 at the concentration contained in the print dot. As shown in Figure 7, the maximum value of the emission intensity of Compound 3 is near 450 nm, and Compound 3 did not absorb at 450 nm or higher. Therefore, the emission of Compound 3 compensates for the region below 500 nm, where the visibility reduction is particularly significant due to light scattering from carbon black. In addition, the emission spectrum is broad, and the emission is emitted up to the long wavelength region near 700 nm, but the emission intensity at 570 nm is small, about 24% of the maximum value. In other words, it was found that the emission intensity is small from 570 nm to 700 nm, which is the long wavelength region of the visible region and the yellow to red light region, and visibility is hardly reduced even if the color of the substrate is brown.

Figure 9 shows a spectrum 16 of the reflected light intensity of a printed dot to which the fluorescent material Compound 3 has been added, and a spectrum 17 of the reflected light intensity of a printed dot to which no fluorescent material has been added. From Figure 7, it is believed that the emission of Compound 3 increases the emission intensity in the region between 400 nm and 500 nm, which has low visibility, thereby improving visibility.

The reason why the visibility of the print in Example 2, in which compound 4 was added as a fluorescent material, was good is as follows. Figure 10 shows the emission spectrum 18 of compound 4 and the absorbance spectrum 19 at the concentration contained in the printed dot. The maximum value of the emission intensity of compound 4 is near 440 nm, and no absorption of compound 4 was confirmed above 420 nm. Therefore, the emission material of compound 4 compensated for the region below 500 nm, where the visibility was significantly reduced due to light scattering from carbon black. In addition, the emission spectrum was sharper than that of compound 3, and no emission was confirmed in the long wavelength region above 520 nm. Therefore, this printed dot was clearer than the printed dot formed in Example 1, in which compound 3 was used.

Figure 11 shows the reflected light intensity spectrum 20 of a printed dot to which compound 4, a fluorescent material, has been added. It is believed that the light emitted by the fluorescent material increases the light emission intensity in the range of 400 nm to 500 nm, where visibility is particularly poor, thereby improving visibility.

The reason why the visibility of the print according to Example 2, in which compound 5 was added as a fluorescent material, was good is as follows. Figure 12 shows the spectrum 21 of compound 5 and the absorbance spectrum 22 at the concentration contained in the print dot. The maximum value of the emission intensity of compound 5 is near 460 nm, and compound 5 did not absorb at 460 nm or more. Therefore, the emission of compound 5 compensated for the region below 500 nm, where the visibility was significantly reduced due to light scattering from carbon black. In addition, the emission spectrum was sharper than that of compound 3, and no emission was confirmed in the long wavelength region above 550 nm. Therefore, the print dot containing compound 5 was clearer than the print dot formed with the ink according to Example 1, which used compound 3. Compound 5 emits light at a short wavelength region of 420 nm, and no emission was observed below this wavelength. Therefore, the emission intensity is low in the region from 400 nm to 440 nm. However, as shown in Figure 1, the visibility of humans decreases as the visibility approaches the short wavelength region beyond 550 nm. To improve visibility, it is desirable for the emission intensity between 440 nm and 500 nm to be stronger than the emission intensity between 420 nm and 440 nm.

Figure 13 shows a spectrum 23 of the reflected light intensity of a printed dot to which the fluorescent material Compound 5 has been added. It is believed that the emission of Compound 5 increases the emission intensity in the 440 nm to 500 nm range, which has a higher visibility than the 400 nm to 440 nm wavelength range, thereby improving visibility.

On the other hand, the reason why the visibility of the print in Comparative Example 1, in which compound 6 was added as a fluorescent material, was low is as follows. Figure 14 shows the emission spectrum 24 of compound 6 and the absorbance spectrum 25 at the concentration contained in the printed dot. The emission intensity of compound 6 was at its maximum near 510 nm, and the emission spectrum was broad up to near 700 nm. Even at 570 nm, the emission intensity was 50% or more of the maximum emission intensity, and yellow to red light was also emitted strongly. Furthermore, compound 6 has absorption from 400 nm to 500 nm. Therefore, compound 6 itself absorbs the purple to blue emission from 400 nm to 500 nm in the emission spectrum of compound 6. From the above results, in the wavelength region of 400 nm to 500 nm in which the emission is particularly reduced by Rayleigh scattering due to carbon black, the emission is suppressed by the absorption of compound 6 itself, and the reflected light intensity of the printed dot is weakened. Furthermore, in the yellow to red region, the reflected light intensity of the printed dots was increased by the emission of compound 6, which is thought to have reduced visibility.

Figure 15 shows spectrum 26 of the reflected light intensity of a printed dot to which compound 6, a fluorescent material, has been added, and spectrum 17 of the reflected light intensity of a printed dot to which no fluorescent material has been added. The reflected light intensity from 400 nm to 500 nm, which is necessary to improve visibility, is lower than when compounds 3, 4, and 5 are used, and the reflected light intensity above 570 nm is high, so it is believed that adding a fluorescent material did not improve visibility.

These results show that the visibility of the ink can be improved by using a fluorescent material whose emission intensity has a maximum in the wavelength range of 400 nm to 500 nm. Furthermore, it was found that the yellow to red emission intensity can be reduced and visibility can be further improved by using a fluorescent material whose emission intensity at 570 nm is 30% or less of the emission intensity at the maximum emission wavelength.

(4-2) Heat Resistance Evaluation Results From Table 2, the printed dots formed with the inks according to Examples 1 to 5, 7 to 9, and Comparative Examples 1 and 2 were not deformed by heat. This shows that the printed dots formed with the inks according to Examples 1 to 5, 7 to 9, and Comparative Examples 1 and 2 have sufficient heat resistance. However, the printed dots formed with the ink according to Example 6 did not have sufficient heat resistance. The printed dots formed with the ink according to Example 6 were circular when viewed from directly above before being left in a thermostatic chamber at 240°C, but after being left in the chamber, the printed dots were thermally melted and became irregular in shape. Furthermore, after being left in a thermostatic chamber at 260°C for 3 minutes, some of the adjacent printed dots were fused together.

The ink in Example 6 contains polyester resin C. Polyester resin C is a dicarboxylic acid unit having carboxy groups at both ends, with adipic acid consisting of a hydrocarbon chain between them. This unit does not have a benzene ring. In addition, the diol unit is neopentyl glycol, which does not have a benzene ring. Therefore, it is believed that a stacking structure could not be formed and the heat resistance was low.

These results show that the heat resistance of prints can be improved by using ink containing polyester resin with a benzene ring.

(4-3) Alcohol Resistance Evaluation Results From Table 2, it was found that the inks according to Examples 1 to 5 and Comparative Examples 1 and 2 had ethanol resistance, with no change in the print even after immersion in ethanol. On the other hand, the inks according to Examples 6 to 9 had faint or disappeared print after immersion in ethanol, and had insufficient ethanol resistance compared to Examples 1 to 5.

The inks in Examples 1 to 5 and 7 and Comparative Examples 1 and 2 contain polyester resins A, B, D, and E. Polyester resins A, B, D, and E are polyester resins that have a benzene ring.

On the other hand, the inks according to Examples 6, 8, and 9 contain polyester resin C, acrylic resin F, and styrene/acrylic resin G, respectively. Since polyester resin C does not have a benzene ring in its structure, it cannot form a stacking structure, and it is believed that the gaps between the molecular chains are larger than those of resins that form a stacking structure. In addition, ethanol molecules can easily enter and exit between the resin molecular chains in the printed dots. Therefore, it is believed that the release of carbon black, which is a pigment, occurs along with ethanol, and the printing of the ink according to Example 6 becomes lighter. In addition, the monomer of acrylic resin F is methyl acrylate, and the main chain is a hydrocarbon chain. In addition, the monomer of styrene/acrylic resin G is styrene and methyl acrylate, and the main chain is a hydrocarbon chain. For this reason, it is believed that when the main chain of the resin used in preparing the ink is a hydrocarbon chain, the ethanol resistance is reduced.

The polyester resin E contained in the ink of Example 7 has a benzene ring, but has a hydroxyl value of 46 mg (KOH)/g and an acid value of 16 mg (KOH)/g, so the sum of the hydroxyl value and acid value exceeds 60 mg (KOH)/g. Therefore, it is believed that the proportion of hydrophilic substituents is high, resulting in reduced ethanol resistance.

On the other hand, among polyester resins A, B, and D, the polyester resin D had the largest sum of hydroxyl value and acid value, at 58 mg (KOH)/g. From Examples 1 to 5 and 7, it was found that when the resin used to prepare the ink is a polyester resin, printing dots with sufficient ethanol resistance can be obtained by setting the sum of hydroxyl value and acid value to 60 mg (KOH)/g or less.

The present invention is not limited to the above-described embodiments, but includes various modified examples. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and are not necessarily limited to those having all of the configurations described. It is also possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. It is also possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

1...Spectrum of reflected light intensity of printed dot 2...Emission spectrum of compound 1 3...Spectrum of reflected light intensity of printed dot 4...Emission spectrum of compound 2 5...Absorbance spectrum at the concentration contained in the printed dot 6...Spectrum of reflected light intensity of printed dot with compound 2 added 7...Spectrum of reflected light intensity of printed dot without compound 2 added 8...Nozzle 9...Ink droplet 10...Charged electrode 11...Deflection electrode 12...Subject to be printed 13...Gutter 14...Emission spectrum of compound 3 15...Absorbance spectrum at the concentration contained in the printed dot containing compound 3 16...Spectrum of reflected light intensity of printed dot with compound 3 added 17: Spectrum of reflected light intensity of printed dots without added fluorescent material 18: Emission spectrum of compound 4 19: Absorbance spectrum at the concentration contained in printed dots containing compound 4 20: Spectrum of reflected light intensity of printed dots with added compound 4 21: Emission spectrum of compound 5 22: Absorbance spectrum at the concentration contained in printed dots containing compound 5 23: Spectrum of reflected light intensity of printed dots with added compound 5 24: Emission spectrum of compound 6 25: Absorbance spectrum at the concentration contained in printed dots containing compound 6 26: Spectrum of reflected light intensity of printed dots with added compound 6

Claims (12)

A conductive agent;
An organic solvent;
Resin and
A pigment including carbon black;
and a fluorescent material having a maximum emission wavelength in the wavelength region of 400 nm or more and 500 nm or less. 2. The ink for an inkjet printer of the charge control system according to claim 1,
1. An ink for a charge-controlling ink-jet printer, comprising: a fluorescent material content in the ink of 0.1 wt % or more and 1.0 wt % or less. 3. The ink for an inkjet printer of the charge control system according to claim 1,
The fluorescent material has an emission intensity at 570 nm that is 30% or less of the emission intensity at the maximum emission wavelength. 4. The ink for an inkjet printer of the charge control system according to claim 1,
1. An ink for a charge-controlling ink-jet printer, wherein the absorbance of said fluorescent material in the ink is smaller than the emission intensity within a wavelength range of 400 nm or more and 500 nm or less. 5. The ink for an inkjet printer of the charge control system according to claim 1,
The ink for a charge control type inkjet printer is characterized in that the resin contains a polyester resin having a benzene ring. 6. The ink for charge-controlling ink-jet printers according to claim 5,
The ink for a charge-controlling ink-jet printer is characterized in that the sum of the acid value and the hydroxyl value of the polyester resin is 60 mg (KOH)/g or less. 7. The ink for a charge-controlling ink-jet printer according to claim 5,
The ink for a charge-controlling ink-jet printer, wherein the polyester resin contains a dicarboxylic acid or diol having a benzene ring, and an aliphatic dicarboxylic acid or aliphatic diol. The ink for a charge control type ink jet printer according to any one of claims 1 to 7,
The ink for a charge-controlling ink-jet printer is characterized in that the average particle size of the pigment is 0.02 μm or more and 0.5 μm or less. 9. The ink for an inkjet printer of the charge control system according to claim 1,
Further comprising a dispersant for the pigment;
1. An ink for a charge-controlling inkjet printer, wherein the dispersant contains a cyclic ester structure in its molecular structure. 10. The ink for an ink-jet printer of the charge control system according to claim 9,
The ink for a charge-controlling ink-jet printer is characterized in that the amount of the dispersant added is 25 wt % or more and 75 wt % or less with respect to the pigment. 10. The ink for an ink-jet printer of the charge control system according to claim 9,
2. An ink for a charge-controlling ink-jet printer, wherein the boiling point of the dispersant is 300° C. or less at normal pressure. The ink for an inkjet printer according to any one of claims 1 to 11,
The ink for a charge control type ink jet printer is characterized in that the organic solvent is a solvent that is not infinitely diluted with water.

Images