JP2692963B2 - Inkjet recording method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to the fixability and printing of ink when printed on a recording paper, which is so-called plain paper, which is sized and has cellulose fibers exposed on the surface. The present invention relates to an inkjet recording method that enables recording with excellent quality.

[Prior Art] Ink for an inkjet printer is generally an aqueous type ink containing dyes and alcohols as main components. Such an ink component has one property (whether or not ink droplets are normally ejected from the nozzle when the first recording signal is given), and stickiness (whether the ink easily hardens at the nozzle tip portion). No.), fixability, storability, color quality, etc., but the most important of these are typewriter paper, wire dot printer paper, electrophotographic paper, which are widely used in offices. Good printing can be performed on various plain papers (non-coated papers) such as word processor papers, letter papers, and report papers.

That is, with conventional ink, even if good print quality is obtained on a certain paper, the dot shape becomes uneven on another type of paper, the fixability of the ink on the paper deteriorates, and There were problems such as the parts being rubbed and dirty, or the ink getting on the hands when the printed matter was touched.

It is considered that the main cause of the irregular dot shapes is that the printed ink spreads irregularly in the voids of the paper fibers and filler. As a result, the periphery of characters and figures becomes unclear, making it difficult to read, and the printing quality deteriorates.

Various improvements have been made for the purpose of improving such problems. For example, JP-A-57-102970 uses an ink whose pH is adjusted to be strongly alkaline, and JP-A-55-65269 uses an ink to which a large amount of a surfactant is added. However, Japanese Patent Application Laid-Open No. 58-13675 describes that an ink containing a water-soluble polymer is used.

However, making the pH of the ink strongly alkaline is dangerous when the ink is touched by hand, and the dot shape and fixability may not be good on neutral paper. Also, when a large amount of a surfactant is added to the ink, the wettability of the ink to the recording paper is improved and the ink permeates faster, resulting in better fixability, but depending on the paper type, A large number of irregular dots are generated, and the ink retreats from the surface of the orifice into the nozzle depending on the driving conditions of the printer head, causing problems such as ink not being ejected.

If a water-soluble polymer is added to the ink, the dot shape and the fixability of the print area will be improved to some extent, but the viscosity of the ink will tend to rise sharply at the tip of the orifice of the printer head. If left in the room for a few minutes, there is a problem in that normal discharge cannot be performed.

[Problem to be Solved by the Invention] Therefore, an object of the present invention is to provide an ink jet recording method which can overcome the problems of the above-mentioned conventional techniques and can obtain good print quality on plain paper.

[Means for Solving the Problems] The above object is achieved by the present invention described below.

That is, in an inkjet recording method in which a droplet of an ink containing a pigment and a liquid medium in which the pigment is dispersed is applied to a recording paper to perform recording, the ink contains carbon black as a pigment and is measured by the Bristow method. The inkjet recording method is characterized in that the absorption coefficient Ka for paper is in the range of ² to 10 ml / m ² · sec ^1/2 .

[Preferred Embodiment] <Type of Inkjet Printer Suitable for Recording on Plain Paper> An inkjet printer for recording on plain paper using a water-based ink preferably has the characteristics shown in Table 1.

It is important for a printer to have a character resolution required for a document and to be adjusted so that the print density, that is, the reflection density of a solid print portion is approximately 1.0 to 1.4.

Generally, the higher the dot density, the better the printing quality. If the dot density is lower than 300 dpi, for example, the volume of the ink droplet becomes too large, and it is difficult to obtain the good printing quality.

Therefore, as shown in Table 1, use a printer in which the dot density and the volume of the ink droplet are constant, that is, a printer in which the dot density is 300 dpi or more and the ink droplet volume is 168 pl or less. It is important that a good quality image is obtained.

<Absorption coefficient> In the present invention, as an ink, an absorption coefficient for recording paper is used.
It is characterized by using an ink whose Ka is 2 ml / m ² · sec ^1/2 ≦ Ka ≦ 10 ml / m ² · sec ^1/2 .

The absorption coefficient is measured by a Bristow Tester according to JAPAN TAPPI UM405 (this method is generally called Bristow method). The absorption coefficient is obtained by putting a certain amount of ink in the head box, transferring the ink to the paper attached around the rotatable disk, and determining the transfer amount, as schematically shown in FIG. To be By changing the rotation speed of the disk, the amount of transferred ink can be measured at a contact time of 0.1 to 2 seconds.

The relationship between the contact time T (sec) ^1/2 of the ink with the paper and the transfer amount V (ml / m ² ) of the ink is as shown in the graph of FIG. 3, and the slope Ka of the graph is the absorption coefficient. Become. In addition, the transfer amount of the ink when the ink contact time is 0 second is defined as the roughness index Kr.
It is called (ml / m ² ), and represents the amount of ink that enters into the irregularities on the surface of the paper. In the initial stage when the ink comes into contact with the paper, there is a time Kw (sec) ^1/2 when the ink is not absorbed for a while. This is called the wetting time, which corresponds to the time until the surface of the paper gets wet with the ink.

When the liquid absorption amount (transfer amount) of the paper is expressed by the formula according to the graph of FIG. 3, it is as follows.

Liquid absorption amount = Kr + Ka (T−Kw) ^1/2 Absorption coefficient Ka is the Lucas-Washburn capillary permeation equation h = (γ cos θ · rt / 2η) ^1/2 (where, h: depth of permeation, γ: Surface tension of liquid, cos θ:
The contact angle between the paper and the liquid, r: the diameter of the pores on the surface of the paper, t: the penetration time, η: the viscosity of the liquid) and the coefficient when t is a parameter. In other words, Ka = (γcosθ · r / 2η) ^1/2 . As is clear from this equation, the absorption coefficient Ka is a value determined by the physical properties of the ink and the wettability between the paper and the ink.

When the value of γ / η is large (that is, when the contact angle of the ink is small), the absorption coefficient Ka is large and the wettability of the ink is good, so that the fixability of the ink is good. Conversely, γ
When the value of / η is small (that is, when the contact angle of ink is large), the absorption coefficient Ka becomes small and the printing quality becomes good.

According to the study by the present inventor, the absorption coefficient Ka is 2 ml / m ² · sec.
^{If it is} less than ^1/2 , it takes 30 seconds or more to dry the printed area due to poor ink fixability, and the absorption coefficient Ka is 10 ml / m ² · sec ^1/2
It was found that when the value exceeds the range, blurring of the edges of the printed part and feathering are conspicuous, and the printing quality is significantly deteriorated. Therefore, it is excellent in fixing the ink to the recording paper,
In addition, in order to keep the printing quality good, the absorption coefficient Ka is 2 ml /
It is important to use ink in the range of m ² · sec ^1/2 ≦ Ka ≦ 10 ml / m ² · sec ^1/2 .

<Recording Paper> In the recording method of the present invention, good printing quality can be obtained by printing on plain paper which is generally widely used. Among them, more preferable plain paper and its physical properties are shown in Table 2. .

As shown in Table 2, the physical properties of plain paper are various, and especially the rosin size, which indicates the wettability to water or water-based ink, and the degree of ink penetration rate. Looking at the Steckigt sizing degree, which shows that the property is not uniform.

Therefore, the inventor of the present invention used a plain paper of No. pp-14 to pp-21 in Table 2 to conduct a water absorption test (JIS-8140) according to the CCBB method.
Based on the Japanese Industrial Standard), the liquid absorption amount of these papers with respect to the liquid medium contained in the ink was measured. The liquid absorption is
It was calculated from the weight of the paper 60 seconds after the application of the liquid medium. The result is shown in FIG.

FIG. 1 shows how the amount of liquid absorbed by the paper changes when the mixing ratio of water, which is a liquid medium, and glycerin is changed. As far as the amount of liquid absorption determined by the COBB method is concerned, all the measured papers showed the same tendency that the amount of liquid absorption decreased as the amount of glycerin increased.

When such a tendency is observed, the liquid is permeated by fiber absorption, that is, the liquid permeation is caused by the absorption of the liquid into the fibers of the paper and the swelling of the fibers, and another mechanism is capillary absorption, In other words, the liquid does not permeate when the liquid penetrates the voids in the paper or the tubes in the fibers (The 4th Non-Impact Printing Symposium (Tokyo, July 23, 1987) was announced. "Plain inkjet suitability (1)" (Sato, Saito, Yokoyama)).

From the results of FIG. 1, the plain paper shown in Table 2 has the same liquid permeation mechanism called fiber absorption with respect to the mixed solvent of water-organic solvent which is the solvent component of the ink used in the inkjet recording method. I understand that I will take it.

From these facts, it can be understood that the influence of the physical properties of the recording paper on the absorption coefficient is small, and the influence of the solvent component of the ink is large. However, the effect of the paper roughness index Kr should be considered. That is, when the roughness index Kr exceeds 14 ml / m ² , the contour of the printed matter is liable to be blurred and the printing quality is likely to be deteriorated. Therefore, the roughness index Kr is preferably 14 ml / m ² or less.

As recording paper that gives improved printing quality, the ash content is 2.86 to 15.2% and the rosin-based sizing agent content is 0.14 to 0.74.
%, The thickness is 81 to 136 μm, the basis weight is 57 to 89 g / m ² , the Steckigt sizing degree is 7 to 71 seconds, and the roughness index Kr is 14 ml / m ² or less.

<Ink> The ink used in the present invention is preferably prepared by dispersing carbon black in water, a water-soluble organic solvent, or a solvent containing ethanol as a main component, and as described above, has an absorption coefficient Ka for recording paper. 2 ml / m ²・ sec ^1/2 ≦ Ka ≦ 10 ml / m ²
・ It satisfies sec ^1/2 .

By using an ink containing a pigment and having an absorption coefficient Ka adjusted to the above range as the inkjet ink, irregular bleeding of the ink droplets is suppressed, and high-density, well-defined dots are formed. It

As the water-soluble organic solvent used, for example, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol 200, polyethylene glycol 300, polyethylene glycol 400, polypropylene glycol 200, polypropylene glycol 300, polypropylene glycol 400, polyhydric alcohols such as glycerin, trimethylolpropane, pentaerythritol, glycol ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and their acetic acid esters, ethylcarbitol, methylcarbyl Carbitols such as tall, butyl carbitol, diethyl carbitol, and vinegar thereof Esters, amines such as triethanolamine and monoethanolamine, formamide, dimethylformamide, dimethylsulfoxide, 1,3-dimethylimidazolidinone, N-methylpyrrolidone, etc. Solvents may be mentioned.

In order to prevent the ink from solidifying at the nozzle tip, it is preferable to contain polyhydric alcohols in an amount of 20% or more. To obtain more stable discharge, add 3-10% ethanol.
It is effective to add.

The main component of the ink used in the present invention comprises the above-mentioned components, but if necessary, various kinds of surfactants, pH adjusters, rust preventives, antiseptics, fungicides, antioxidants, chelating agents, etc. You may add the additive of.

As a method for producing the ink, first, a pigment is added to an aqueous solution containing at least a dispersion resin, an amine, and water, the mixture is stirred, then dispersed, and if necessary, a centrifugal separation treatment is performed to obtain a desired dispersion liquid. Next, the above-exemplified components are added to this dispersion, and the mixture is stirred to form an ink.

[Examples] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. All% in the text are based on weight.

Examples 1 to 4 carbon black (trade name: MCF88, manufactured by Mitsubishi Kasei) 5% styrene-acrylic acid copolymer 0.6% ethanolamine 0.1% thiodiglycol 10% ethylene glycol 10% ethanol 10% water 64.3% the above inks Using plain papers No. pp-21 to pp-24 in Table 2, the absorption coefficient Ka of the ink and the roughness index Kr of the paper were measured by a Bristow tester (manufactured by Toyo Seiki). Further, using these plain paper and ink, printing was carried out using a type 1 printer shown in Table 1, and printing was evaluated.
Table 3 shows the results. The print evaluation was based on the following criteria.

(1) Print quality Printed materials consisting of alphabets and numbers were shown to 20 men and women for sensory evaluation. The sensory evaluation was based on the following criteria based on a 3-point perfect score system, and was classified into three grades A to C according to the total score.

 3 points: A sharp image with sharp edges.

2 points: An image in which the shape of the character is good.

 1 point: Feathering and bleeding occur.

A: 100 points or more B: 80 to 99 points C: 79 points or less (2) Ink fixing time Printing is performed at 1 second intervals for 30 seconds, then sillbon paper is placed on the printed material and a weight is applied at a constant speed from the top. The printed matter was checked for rubbing. 5-30 after printing by this method
The ink fixing property for 2 seconds was examined.

(3) Optical Density of Printed Product The printed product was measured using a Macbeth densitometer (RD918).

Examples 5 to 9 Carbon black (trade name: MA100, manufactured by Mitsubishi Kasei) 5% Styrene-acrylic acid copolymer 0.6% Ethanolamine 0.1% Polyethylene glycol (molecular weight 300) 10% Ethylene glycol 10% Ethanol 10% Water 64.3% The above inks and No. pp-3 to pp-21 to pp-24 in Table 2
Printing was performed by using the type 3 printer shown in Table 1 using plain paper up to the above, and printing evaluation was performed in the same manner as in Example 1. Table 3 shows the results.

Comparative Examples 1 and 2 Carbon black (trade name: MCF88, manufactured by Mitsubishi Kasei) 5% Styrene-acrylic acid copolymer 0.6% Ethanolamine 0.1% Glycerin 10% Ethanol 10% Water 74.3% The above ink and No. 2 in Table 2. Printing was performed using plain paper of .pp-16 and pp-20 using the type 1 printer shown in Table 1, and the printing evaluation was performed in the same manner as in Example 1. Table 3 shows the results. Feathering did not occur in the obtained printed matter, but the lines were extremely fine because there was little ink bleeding.

Comparative Examples 3 and 4 Carbon black (trade name: MCF88, manufactured by Mitsubishi Kasei) 5% Styrene-acrylic acid copolymer 0.6% Ethanolamine 0.1% Diethylene glycol 50% Ethanol 10% Water 34.3% The above ink and No. Printing was performed using plain paper of .pp-16 and pp-20 using the type 3 printer shown in Table 1, and the printing evaluation was performed in the same manner as in Example 1. Table 3 shows the results. The obtained printed matter had feathering and blurring of the edge portion, and the print quality was extremely poor.

[Effects of the Invention] As described above, according to the present invention, as an ink,
By using the one with the absorption coefficient Ka for recording paper adjusted to 2-10 ml / m ² · sec ^1/2 , it is possible to obtain an image with good print quality and excellent ink fixability. .

Further, according to the present invention, by adjusting the absorption coefficient of the ink within the above range, both feathering and bleeding are less likely to occur, so that good printing can be performed on various widely spread office papers. In addition, since it is a pigment ink, it is also excellent in fastness such as light resistance, water resistance, and ozone resistance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing changes in the liquid absorption amount of recording paper when the mixing ratio of water and glycerin is changed, and FIG. 2 measures the absorption coefficient of ink. FIG. 3 shows a device for
The figure is a diagram showing the relationship between the time during which the ink contacts the recording paper and the amount of transfer of the ink to the recording paper.

Claims (4)

(57) [Claims] 1. An inkjet recording method for recording by applying droplets of an ink containing a pigment and a liquid medium in which the pigment is dispersed onto a recording paper, wherein the ink contains carbon black as a pigment and is measured by the Bristow method. An ink jet recording method characterized in that the absorption coefficient Ka for recording paper is in the range of ² to 10 ml / m ² · sec ^1/2 . 2. The ink jet recording method according to claim 1, wherein the roughness index Kr of the recording paper is 14 ml / m ² or less. 3. The ink jet recording method according to claim 1, wherein the recording paper is plain paper. 4. The recording paper has an ash content of 2.86 to 15.2.
%, The amount of rosin-based sizing agent is 0.14 to 0.74%, and the thickness is 81 to 13
6 μm, basis weight 57-89 g / m ² , Steckigt sizing degree 7-7
The ink jet recording method according to claim 1, wherein a recording paper satisfying a condition that the roughness index Kr is 14 ml / m ² or less for 1 second is used.