JP7175673B2 - Inkjet recording method and inkjet recording apparatus - Google Patents

Description

The present invention relates to an inkjet recording method and an inkjet recording apparatus.

In recent years, the frequency of use of inkjet recording apparatuses in offices is increasing. In addition, the ink jet recording apparatus is required to have a higher recording speed. In order to improve the printing speed, a conventional serial printing head (serial head) is used to print an image in so-called multi-pass printing, whereas a line-type printing head (line head) is used to print an image in a so-called 1-pass printing method. In some cases, a method of recording an image with

Of the recording devices used in offices, electrophotographic recording devices generally eject the recording medium (recorded matter) with the image printed on it with the back side facing up. is. For example, when two pages of text are recorded on one side of each of two recording media and are ejected from the reverse side, the first page is ejected with the recording surface facing down, and the second page is ejected. Eye recordings are ejected onto it. Therefore, by turning over the stack of printed materials in the discharged state, it is possible to obtain a stacked stack of printed materials arranged in the order of the pages. On the other hand, in the case of so-called surface ejection, in which printed matter is ejected with the front side facing upward, the page order of the resulting stack of printed matter is reversed. Therefore, in the case of a recording apparatus for office use, in which a large number of recorded materials are often recorded at one time, the reverse side ejection is often adopted from the viewpoint of usability.

If you use a conventional inkjet recording device to record an image on a recording medium such as plain paper that does not have a coat layer, it will take several hours to several days after recording with the ink-applied side inside. So-called curl (plus curl), in which the medium is deformed into a concave shape, tends to occur. In order to suppress the occurrence of such curling, an ink containing a specified water-soluble organic compound has been proposed (Patent Document 1). In addition, it has been proposed to use high-viscosity ink of 5 mPa·s or more in an ink jet recording method, and recording of an image by surface discharge is disclosed (Patent Document 2).

JP 2005-298813 A Japanese Patent Application Laid-Open No. 2005-320509

For the following two reasons, conventional ink jet recording apparatuses have widely adopted front side paper ejection instead of rear side paper ejection. First, inkjet recording devices are mainly used for outputting printed matter (postal items such as New Year's cards, photographs, etc.) that do not require consideration of the printing order, and printed matter with a small number of pages that does not require much sorting. It is mentioned that Further, in the case of an inkjet recording method using water-based ink, from the viewpoint of drying the ink quickly, it is advantageous to discharge the paper with the immediately preceding recording surface facing upward. On the other hand, as described above, the back-side discharge has the advantage that "a bundle of recorded materials can be obtained in which the pages are arranged and stacked in order".

Even in the case of using the inkjet recording method, the use of reverse paper ejection is now being considered for office recording devices, etc., assuming applications in which a large number of recorded materials are recorded at once. there is In addition, by adopting the back side paper ejection mechanism, it is possible to arrange the recording section and the paper ejection section above the storage section for the recording medium, which is advantageous in reducing the installation area of the apparatus.

The inventors of the present invention applied the ink having the composition proposed in Patent Document 1 to an ink jet recording apparatus having a back surface discharge mechanism, and conducted various studies. As a result, it was found that, unlike the positive curl described above, curling (negative curling), in which the recording medium is deformed with the ink-applied surface facing outward, is likely to occur immediately after printing. In addition, it was found that negative curl may occur even when the ink having the composition proposed in Patent Document 2 is applied.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an ink jet recording method that employs back-side ejection, and which is capable of obtaining recorded matter in which the occurrence of negative curl is suppressed. Another object of the present invention is to provide an inkjet recording apparatus for use in this inkjet recording method.

That is, according to the present invention, the inkjet recording method includes a recording step of recording an image on a recording medium having a first side and a second side by ejecting a water-based ink containing a water-soluble organic solvent from a recording head. and further comprising, after the recording step, a paper discharging step of discharging the paper with the later recorded surface of the first surface and the second surface facing downward in the direction of gravity, wherein the viscosity η of the water-based ink ₁ (mPa s) is 5.0 mPa s or more and 15.0 mPa s or less , and the viscosity η ₂ (mPa s) of the evaporated ink obtained by evaporating 20% by mass of the water-based ink and the water-based ink The difference η ₂ −η ₁ of the viscosity η ₁ (mPa·s) of the water-based ink is 4.0 mPa·s or more and 15.0 mPa·s or less , and the water-based ink contains a polyhydric alcohol compound that is solid at 25° C. and the content (% by mass) of the polyhydric alcohol compound in the aqueous ink is 10.0% by mass or more based on the total mass of the ink . .

According to the present invention, it is possible to provide an ink jet recording method that employs back side ejection, and that can obtain a recorded matter in which the occurrence of negative curl is suppressed. Further, according to the present invention, it is possible to provide an inkjet recording apparatus used for this inkjet recording method.

1 is a cross-sectional view showing an embodiment of an inkjet recording apparatus of the present invention; FIG. FIG. 3 is a cross-sectional view for explaining a conveying path of a recording medium when ejecting the back side of the recording medium; FIG. 3 is a cross-sectional view for explaining a conveying path of a recording medium when ejecting the back side of the recording medium; FIG. 3 is a cross-sectional view for explaining a conveying path of a recording medium when ejecting the back side of the recording medium; FIG. 10 is a cross-sectional view for explaining a conveying path of a recording medium during surface discharge; FIG. 10 is a cross-sectional view for explaining a conveying path of a recording medium during surface discharge; FIG. 10 is a cross-sectional view for explaining a conveying path of a recording medium during surface discharge; FIG. 10 is a cross-sectional view for explaining a conveying path of a recording medium during surface discharge;

The present invention will be further described in detail below with reference to preferred embodiments. In the present invention, when the compound is a salt, the salt is dissociated into ions in the ink, but for convenience, it is expressed as "containing the salt." In addition, water-based ink for inkjet is sometimes simply referred to as "ink". Unless otherwise specified, physical property values are values at normal temperature (25° C.) and normal pressure (1 atm).

As described above, the inventors applied the ink having the composition proposed in Japanese Patent Application Laid-Open No. 2002-300001 to an inkjet recording apparatus having a back surface ejection mechanism. As a result, it was found that negative curl occurred immediately after recording. In addition, it has been found that negative curling is more likely to occur when high-speed recording is performed using an inkjet recording apparatus equipped with a line head together with a back side discharge mechanism. As the average speed (linear velocity) at which the recording medium passes through the portion facing the recording head in the conveying path of the recording medium increases, negative curling occurs more remarkably. Then, once negative curl occurs during continuous recording, the recorded matter discharged first to the paper discharge tray interferes with the recorded matter discharged later. As a result, jams occur and the printed materials are not stacked in order.

The cause of minus curl is considered to be different from the cause of plus curl. First, the difference between the negative curl that occurs when using an ink jet recording apparatus having a back surface ejection mechanism and the positive curl that has been a problem in the past will be described.

When the ink is applied to the recording medium, the liquid components (water and water-soluble organic solvent) in the ink break hydrogen bonds between celluloses constituting the recording medium and swell the cellulose. As a result, a minus curl occurs in which the recording medium is deformed into a convex shape (concave shape when viewed in the ejected state) with the ink-applied surface facing outward. This minus curl occurs immediately after recording and stops in several tens of seconds.

On the other hand, plus curl occurs as follows. When the ink is applied to the recording medium, cellulose forming the recording medium swells. Among the liquid components in water-based ink, water usually has a high vapor pressure, so water begins to evaporate before the water-soluble organic solvent. As the water that swells the cellulose evaporates, the broken hydrogen bonds between the celluloses are formed again, causing the cellulose to shrink. As a result, a positive curl occurs in which the recording medium is deformed into a concave shape with the ink-applied surface facing inward. That is, plus curl occurs due to evaporation of water and shrinkage of cellulose, and progresses slowly over a period of hours to days after recording.

Considering the above points, it is considered that even in the case of using a conventional ink jet recording apparatus equipped with a surface discharge mechanism, there is a force that causes negative curling. However, in the case of an inkjet recording apparatus equipped with a surface ejection mechanism, since the paper is ejected with the recording surface facing up, the self-weight of the recording medium is the force that causes the ink-applied surface to face outward and deforms into a convex shape. Better than Therefore, minus curl hardly occurs. On the other hand, in the case of an ink jet recording apparatus having a back side discharge mechanism, the force that causes the recording medium to swell directly becomes the force that causes the minus curl, and thus the minus curl occurs significantly. Further, when the linear velocity is low, it usually takes several seconds to several tens of seconds from immediately after the ink is applied to the recording medium until the ejection of the recording medium is completed. For this reason, the recording medium swells the moment the ink is applied, but since both ends of the recording medium are nipped by rollers or the like, when the paper is ejected, there is almost no negative curl. there is

The inventors of the present invention conducted various studies to suppress the negative curl that tends to occur peculiarly when using an ink jet recording apparatus having a back surface ejection mechanism. The hydroxy group of the liquid component in the ink enters between the celluloses whose hydrogen bonds have been cut, causing the cellulose to swell, thereby causing minus curl. The present inventors verified whether or not the type of water-soluble organic solvent causes a difference in the swelling rate of a recording medium such as plain paper having no coat layer. Specifically, a certain amount of a water-soluble organic solvent or water generally used for water-based ink for inkjet was applied to a recording medium, and the swelling rate of the recording medium was measured. As a result, it was found that water had the highest swelling rate, and that the swelling rate varied depending on the type of water-soluble organic solvent. Also, the relationship between various physical properties of the water-soluble organic solvent and the swelling rate of the recording medium was verified. As a result, it was found that the higher the evaporation viscosity of the water-soluble organic solvent, the smaller the swelling rate of the recording medium. Ink using a water-soluble organic solvent with a small swelling rate of the recording medium is used in an ink jet recording apparatus having a back paper discharge mechanism rather than ink using a water-soluble organic solvent with a high swelling rate of the recording medium. It was confirmed that the occurrence of negative curl can be suppressed in the case.

As described above, among liquid components used in water-based ink, water has the highest swelling rate of the recording medium. Therefore, in order to suppress the occurrence of negative curl, it is important to prevent water from penetrating the cellulose constituting the recording medium as much as possible. A water-soluble organic solvent that coexists with water tends to have a higher viscosity when evaporated than water, and thus permeates the recording medium slower than water. On the other hand, the moment the ink is applied, water begins to permeate the recording medium and evaporate. As a result, the penetration of the water-soluble organic solvent into the recording medium is delayed, and the evaporation of water proceeds. Then, it is considered that the swelling rate of the recording medium is reduced by reducing the amount of water that permeates the recording medium.

The present inventors examined how much the liquid component in the ink is reduced during the several tens of seconds from when the ink is applied to the recording medium and when negative curling starts to stop. . Specifically, various types of recording media (plain paper) were used under the conditions of 25° C. and 50% relative humidity, assuming a general indoor environment. As a result, it was confirmed that the progress of the negative curl was settled in about 60 seconds after the ink was applied to the recording medium. Further, by periodically measuring the weight of the printed matter for 60 seconds after applying the ink to the recording medium, the amount of the liquid component that evaporates during this period was estimated. It was found that approximately 20% by weight of the liquid component was evaporated. From these results, if the amount of the liquid component that evaporates while negative curl occurs and progresses, it is possible to accurately grasp the situation in which negative curl occurs and progresses.

Considering the above points, by increasing the viscosity of the ink in which the liquid component has evaporated, it is possible to suppress the negative curl that tends to occur when using an inkjet recording apparatus equipped with a back surface ejection mechanism. Therefore, the present inventors further studied the relationship between the viscosity of ink (evaporated ink) in which a certain amount of liquid component has evaporated and the degree of occurrence of negative curl. As a result, the inventors have found that by satisfying the requirements (i) and (ii) shown below, it is possible to suppress the occurrence of negative curl even when the reverse side ejection is employed.
(i) The ink viscosity η ₁ (mPa·s) is 5.0 mPa·s or more.
(ii) The difference η ₂ −η ₁ between the viscosity η ₂ (mPa·s) of the evaporated ink obtained by evaporating 20% by mass of the ink and the viscosity η ₁ (mPa·s) of the ink is 4.0 mPa·s or more. is.

If the viscosity η ₁ of the ink is less than 5.0 mPa·s, even if the viscosity of the evaporated ink is increased, the occurrence of negative curl cannot be suppressed. Even if the ink viscosity η ₁ is 5.0 mPa·s or more, the difference η ₂ -η ₁ between the vaporized ink viscosity η ₂ (mPa·s) and the ink viscosity η ₁ (mPa·s) is If it is less than 4.0 mPa·s, it is not possible to suppress the generation of negative curl.

Conventional common water-based inks are often designed so that the viscosity does not increase much even if the liquid component evaporates to some extent, taking into consideration the effect on ejection characteristics. In this case, η ₂ -η ₁ is at most about 3.0 mPa·s. That is, it can be said that the ink used in the ink jet recording method of the present invention is a type of water-based ink that tends to thicken due to evaporation.

If the difference η ₂ −η ₁ (viscosity difference η ₂ −η ₁ ) between the viscosity η ₂ (mPa·s) of the evaporated ink and the viscosity η ₁ (mPa·s) of the ink increases, the It has been found that the surface opposite to the recording surface of the recorded matter stacked on the paper tray tends to be stained. No image is recorded on the back side (upper side) of the first sheet of recording medium placed on the output tray by back side ejection, or an image is recorded after a certain amount of time has passed since the beginning of double-sided recording. It is Therefore, the recording surface (lower surface) of the second recording medium is ejected while rubbing against the back surface (upper surface) of the first recording medium on the ejection tray. If the viscosity difference η ₂ −η ₁ is too large, the ink applied to the second recording medium will come into contact with the back surface of the first recording medium before it has sufficiently permeated the recording medium. . For this reason, it has been found that a so-called "smear" phenomenon, in which part of the ink applied to the second sheet of recording medium smears the first sheet of recording medium, tends to occur.

If the viscosity η ₁ of the ink is 5.0 mPa·s or more and the viscosity difference η ₂ −η ₁ is more than 10.0 mPa·s, smearing is likely to occur. Further, even if the viscosity difference η ₂ −η ₁ is 10.0 mPa·s or less, smearing is likely to occur if the ink viscosity η ₁ exceeds 10.0 mPa·s. That is, in the ink jet recording method of the present invention that employs the back side ejection, the ink viscosity η ₁ is 10.0 mPa·s or less and the viscosity difference η ₂ −η ₁ is 10.0 mPa·s or less. is preferable because it can suppress the occurrence of smear.

<Inkjet recording method and inkjet recording apparatus>
The inkjet recording method of the present invention (hereinafter also simply referred to as "recording method") has a recording step of ejecting water-based ink from a recording head to record an image on a recording medium having a first side and a second side. recording method. The recording method of the present invention further includes, after the recording step, a paper ejection step of ejecting the paper with the later recorded side of the first side and the second side facing downward in the direction of gravity. The viscosity η ₁ (mPa·s) of the water-based ink is 5.0 mPa·s or more, and the viscosity η ₂ (mPa·s) of the evaporated ink obtained by evaporating 20% by mass of the water-based ink and The difference η ₂ -η ₁ in viscosity η ₁ (mPa·s) is 4.0 mPa·s or more.

Further, the inkjet recording apparatus of the present invention (hereinafter also simply referred to as "recording apparatus") ejects water-based ink from a recording head to record an image on a recording medium having a first side and a second side. equipment used. The recording apparatus of the present invention is provided with a back surface ejection mechanism that ejects the sheet with the surface printed later (immediately before ejection) of the first surface and the second surface facing downward in the direction of gravity. The viscosity η ₁ (mPa·s) of the water-based ink is 5.0 mPa·s or more, and the viscosity η ₂ (mPa·s) of the evaporated ink obtained by evaporating 20% by mass of the water-based ink and The difference η ₂ -η ₁ in viscosity η ₁ (mPa·s) is 4.0 mPa·s or more. Hereinafter, details of the inkjet recording apparatus and the inkjet recording method of the present invention will be described with reference to the drawings.

FIG. 1 is a cross-sectional view showing one embodiment of the inkjet recording apparatus of the present invention. In FIG. 1, x indicates the horizontal direction, y indicates the arrangement direction (depth direction) of the ejection openings of the recording head 8, and z indicates the vertical direction. The ink jet recording apparatus 1 of the embodiment shown in FIG. 1 is in a so-called standby state in which neither the recording operation nor the reading operation is performed.

The inkjet recording apparatus 1 shown in FIG. 1 is a multi-function machine including a recording unit 2 and a scanner unit 3, and various processes related to recording operation and reading operation are executed by the recording unit 2 and the scanner unit 3 individually or in conjunction with each other. do. The scanner unit 3 reads (scans) an original. The inkjet recording apparatus of the present invention may be of a form that does not include a scanner section.

In the recording unit 2, a first cassette 5A and a second cassette 5B for storing cut sheet-shaped recording media S are detachably installed at the bottom of the housing 4 in the vertical direction. The first cassette 5A accommodates relatively small recording media up to A4 size in a flat stack. The second cassette 5B accommodates relatively large recording media up to A3 size in a flat stack. In the vicinity of the first cassette 5A, a first feeding unit 6A that separates and feeds the contained recording media S one by one is provided. Similarly, a second feeding unit 6B is provided near the second cassette 5B. When the recording operation is performed, the recording medium S is selectively fed from one of the cassettes.

The transport roller 7, discharge roller 12, pinch roller 7a, spur 7b, guide 18, inner guide 19, and flapper 11 are a transport mechanism for guiding the recording medium S in a predetermined direction. The transport rollers 7 are drive rollers arranged upstream and downstream of the recording head 8 and driven by a transport motor (not shown). The pinch roller 7 a is a driven roller that rotates while nipping the recording medium S together with the transport roller 7 . The discharge roller 12 is a drive roller arranged downstream of the transport roller 7 and driven by a transport motor (not shown). The spur 7b, together with the transport roller 7 and the discharge roller 12 arranged on the downstream side of the recording head 8, pinches and transports the recording medium S. As shown in FIG.

The guide 18 is provided on the transport path of the recording medium S and guides the recording medium S in a predetermined direction. The inner guide 19 is a member extending in the y direction and has a curved side surface, and guides the recording medium S along the side surface. The flapper 11 is a member for switching the direction in which the recording medium S is conveyed during the double-sided recording operation. The ejection tray 13 is a tray for stacking and holding the recording medium S ejected by the ejection roller 12 after the printing operation is completed.

The print head 8 is a full line type print head, and a plurality of ejection openings for ejecting ink according to print data are arranged in a range covering the width of the print medium S along the y direction. When the recording head 8 is at the standby position, the discharge port surface 8a of the recording head 8 is capped by the cap unit 10 as shown in FIG. When performing a recording operation, the orientation of the recording head 8 is changed so that the ejection port surface 8 a faces the platen 9 . The platen 9 is composed of a flat plate extending in the y-direction, and supports the recording medium S from the rear surface when the recording head 8 performs a recording operation.

The ink tank unit 14 contains four types of ink to be supplied to the recording head 8, respectively. The ink supply unit 15 is provided in the middle of the flow path connecting the ink tank unit 14 and the recording head 8, and adjusts the flow rate of the ink to the recording head 8 within an appropriate range. The maintenance unit 16 includes the cap unit 10 and the wiping unit 17 and operates them at predetermined timings to perform maintenance operations for the recording head 8 .

When the inkjet recording apparatus shown in FIG. 1 is used, the recording medium S accommodated in the first cassette 5A or the second cassette 5B is first conveyed through the apparatus, and an image is recorded by the recording head 8 of the recording section 2. . The recording medium S on which an image has been recorded is discharged to the discharge tray 13 with the side recorded later (immediately before discharge) facing downward in the direction of gravity, that is, turned upside down.

2A to 2C are cross-sectional views for explaining the conveying path of the recording medium when ejecting the back side of the sheet. The recording medium S loaded as the first sheet from the top of the first cassette 5A is separated from the recording medium after the second sheet by the first feeding unit 6A, and while being nipped between the conveying roller 7 and the pinch roller 7a, It is conveyed toward the recording area P between the platen 9 and the recording head 8 . 2A shows the state immediately before the leading edge of the recording medium S reaches the recording area P. FIG. The traveling direction of the recording medium S is changed from the horizontal direction (x direction) to a direction inclined by about 45 degrees with respect to the horizontal direction while being fed to the first feeding unit 6A and reaching the recording area P. be.

In the recording area P, ink is ejected toward the recording medium S from a plurality of ejection openings provided in the recording head 8 . Examples of ink ejection methods include a thermal method in which an ejection element that generates thermal energy is used to generate air bubbles to eject ink, and a so-called piezo method in which ink is ejected by applying mechanical energy to the ink. be able to. In the present invention, it is particularly preferable to use a line head that ejects ink by a thermal method. A platen 9 supports the back surface of the recording medium S in the area to which the ink is applied. Therefore, the distance between the ejection port surface and the recording medium S is kept constant. The recording medium S to which ink has been applied passes through the left side of the flapper 11 whose tip is inclined to the right while being guided by the transport roller 7 and the spur 7b, and along the guide 18, upward in the vertical direction of the inkjet recording apparatus 1. be transported. FIG. 2B shows a state in which the leading edge of the recording medium S passes through the recording area P and is transported vertically upward. The traveling direction of the recording medium S is changed vertically upward by the conveying roller 7 and the spur 7b from the position of the recording area P which is inclined by about 45 degrees with respect to the horizontal direction.

After being transported vertically upward, the recording medium S is discharged to the discharge tray 13 by the discharge roller 12 and the spur 7b. FIG. 2C shows a state in which the leading edge of the recording medium S passes through the ejection roller 12 and is ejected to the ejection tray 13 in the state of back ejection. The discharged recording medium S is held on the discharge tray 13 with the surface on which the image is recorded by the recording head 8 facing downward.

3A to 3D are cross-sectional views for explaining the conveying path of the recording medium during surface discharge. In the case of surface discharge, after an image is recorded by a method similar to the method described with reference to FIGS. 2A to 2C, the recording medium is reversed and discharged within the apparatus. Since the conveying process for recording an image is the same as that shown in FIGS. 2A to 2C, the description is omitted here. The transfer process after FIG. 2C will be described below with reference to FIGS. 3A to 3D.

When the recording operation by the recording head 8 is completed and the trailing edge of the recording medium S passes the flapper 11 , the conveying roller 7 is reversely rotated to convey the recording medium S into the ink jet recording apparatus 1 . At this time, the flapper 11 is controlled by an actuator (not shown) so that its tip is tilted to the left. transported vertically downward. 3A shows a state in which the leading edge of the recording medium S (the trailing edge in the recording operation on the first side) passes the right side of the flapper 11. FIG. After that, the recording medium S is transported along the curved outer peripheral surface of the inner guide 19 and transported again between the recording head 8 and the platen 9 .

The transport path thereafter is the same as in the case of recording an image on the first surface shown in FIGS. 2B and 2C. FIG. 3C shows a state in which the leading edge of the recording medium S passes through the recording area P and is transported vertically upward. At this time, the flapper 11 is controlled by an actuator (not shown) so that the tip thereof is tilted to the right. FIG. 3D shows a state in which the leading edge of the recording medium S passes through the ejection roller 12 and is ejected to the ejection tray 13 in the surface ejection state.

By using the inkjet recording apparatus 1 of the present embodiment, it is possible to perform so-called double-sided recording, in which images are recorded on the first side and the second side (both sides) of a recording medium. In the case of FIGS. 3A to 3D, after an image is recorded on the first side (front side), an image is recorded on the second side (back side). Since the conveying process for recording an image on the first side is the same as that shown in FIGS. 2A to 2C, the description is omitted here. The transfer process after FIG. 2C will be described below with reference to FIGS. 3A to 3D.

When the recording operation on the first side by the recording head 8 is completed and the trailing edge of the recording medium S passes the flapper 11, the conveying roller 7 is rotated in the reverse direction to convey the recording medium S into the ink jet recording apparatus 1. . At this time, the flapper 11 is controlled by an actuator (not shown) so that its tip is tilted to the left. Transported vertically downward. 3A shows a state in which the leading edge of the recording medium S (the trailing edge in the recording operation on the first side) passes the right side of the flapper 11. FIG. After that, the recording medium S is transported along the curved outer peripheral surface of the inner guide 19 and transported to the recording area P between the recording head 8 and the platen 9 again. At this time, the second surface of the recording medium S faces the ejection port surface 8 a of the recording head 8 . FIG. 3B shows the conveying state immediately before the leading edge of the recording medium S reaches the recording area P for the recording operation on the second surface.

The transport path thereafter is the same as in the case of recording an image on the first surface shown in FIGS. 2B and 2C. FIG. 3C shows a state in which the leading edge of the recording medium S passes through the recording area P and is transported vertically upward. At this time, the flapper 11 is controlled by an actuator (not shown) so that the tip thereof is tilted to the right. FIG. 3D shows a state in which the leading edge of the recording medium S passes through the ejection roller 12 and is ejected to the ejection tray 13 .

As the recording medium, a recording medium having no coat layer such as plain paper or uncoated paper, or a permeable recording medium such as a recording medium having a coat layer such as glossy paper or art paper can be used. preferable. Among them, a recording medium having a paper base is preferable, and a recording medium having a paper base without a coat layer is more preferable.

(ink)
The ink used in the recording method of the present invention is a water-based ink for inkjet. The viscosity η ₁ (mPa·s) of this ink is 5.0 mPa·s or more. Further, the difference η ₂ −η ₁ between the viscosity η ₂ (mPa·s) of the evaporated ink obtained by evaporating 20 mass % of this ink and the viscosity η ₁ (mPa·s) of the ink is 4.0 mPa·s or more. is. The ink used in the recording method of the present invention does not have to be a so-called "curable ink". Therefore, the ink used in the recording method of the present invention may not contain compounds such as polymerizable monomers that can be polymerized by the application of external energy. Each component constituting the ink and physical properties of the ink will be described in detail below.

[Color material]
A pigment or a dye can be used as the coloring material. The content (% by mass) of the coloring material in the ink is preferably 0.5% by mass or more and 15.0% by mass or less, and 1.0% by mass or more and 10.0% by mass or less, based on the total mass of the ink. More preferably:

Specific examples of pigments include inorganic pigments such as carbon black and titanium oxide; and organic pigments such as azo, phthalocyanine, quinacridone, isoindolinone, imidazolone, diketopyrrolopyrrole and dioxazine. Among them, carbon black; azo pigments; phthalocyanine pigments; quinacridone pigments and the like are preferred. C. I. Preferred specific examples of numbered organic pigments are listed below. Examples of azo pigments include C.I. I. Pigment Yellow 74 and the like can be mentioned. Examples of phthalocyanine pigments include C.I. I. Pigment Blue 15:3, C.I. I. Pigment Blue 15:4, C.I. I. Pigment Blue 15:6 and the like can be mentioned. Examples of quinacridone pigments include C.I. I. Pigment Violet 19, C.I. I. Pigment Red 122, C.I. I. In addition to Pigment Red 202, solid solutions of multiple types of quinacridone pigments can be used.

Examples of the method of dispersing the pigment include a resin-dispersed pigment using a resin as a dispersant, and a self-dispersing pigment in which a hydrophilic group is bonded to the particle surface of the pigment. Moreover, a resin-bonded pigment in which an organic group containing a resin is chemically bonded to the particle surface of the pigment, or a microcapsule pigment in which the surface of the pigment particle is coated or encapsulated with a resin or the like can be used.

As the resin dispersant for dispersing the pigment in the aqueous medium, it is preferable to use one that can disperse the pigment in the aqueous medium by the action of an anionic group. As the resin dispersant, a resin as described later, especially a water-soluble resin can be used. The content (% by mass) of the pigment in the ink is preferably 0.3 times or more and 10.0 times or less as a mass ratio with respect to the content of the resin dispersant.

As the self-dispersing pigment, those in which an anionic group such as a carboxylic acid group, a sulfonic acid group, or a phosphonic acid group is bonded directly to the particle surface of the pigment or through another atomic group (-R-) is used. be able to. The anionic group may be in either an acid form or a salt form, and in the case of a salt form, it may be in a partially dissociated state or in a completely dissociated state. When the anionic group is in the salt form, cations that serve as counter ions include alkali metal cations, ammonium and organic ammonium. Specific examples of other atomic groups (-R-) include linear or branched alkylene groups having 1 to 12 carbon atoms; arylene groups such as phenylene groups and naphthylene groups; carbonyl groups; imino groups; amide groups; groups; ester groups; ether groups, and the like. Moreover, the group which combined these groups may be sufficient.

As the dye, it is preferable to use one having an anionic group. Specific examples of dyes include dyes such as azo, triphenylmethane, (aza)phthalocyanine, xanthene, and anthrapyridone.

A pigment is preferably used as the coloring material. Among them, it is more preferable to use a self-dispersed pigment or a resin-dispersed pigment using a water-soluble resin as a dispersant.

[resin]
The ink can contain a resin. The resin content (% by mass) in the ink is preferably 0.1% by mass or more and 20.0% by mass or less, and 0.5% by mass or more and 15.0% by mass or less, based on the total mass of the ink. is more preferable.

The resin can be added to the ink to (i) stabilize the dispersed state of the pigment, that is, as a resin dispersant or as an adjunct thereto. In addition, (ii) it can be added to the ink in order to improve various characteristics of the recorded image. Forms of the resin include block copolymers, random copolymers, graft copolymers, combinations thereof, and the like. Moreover, the resin may be a water-soluble resin that is soluble in an aqueous medium, or resin particles that are dispersed in an aqueous medium. Resin particles do not need to contain a coloring material.

As used herein, the phrase "the resin is water-soluble" means that when the resin is neutralized with an alkali equivalent to its acid value, it is water-soluble in a state in which particles whose particle size can be measured by a dynamic light scattering method are not formed. means present in the medium. Whether or not the resin is water-soluble can be determined according to the method described below. First, a liquid (resin solid content: 10% by mass) containing a resin neutralized with an acid value equivalent alkali (sodium hydroxide, potassium hydroxide, etc.) is prepared. Next, the prepared liquid is diluted 10-fold (by volume) with pure water to prepare a sample solution. Then, when the particle size of the resin in the sample solution is measured by the dynamic light scattering method, if particles having the particle size are not measured, it can be determined that the resin is water-soluble. The measurement conditions at this time can be, for example, as follows.

[Measurement condition]
SetZero: 30 seconds Number of measurements: 3 Measurement time: 180 seconds

As the particle size distribution analyzer, a particle size analyzer using dynamic light scattering (for example, trade name “UPA-EX150” manufactured by Nikkiso Co., Ltd.) can be used. Of course, the particle size distribution measuring device and measurement conditions to be used are not limited to those described above.

The acid value of the water-soluble resin is preferably 100 mgKOH/g or more and 250 mgKOH/g or less. The acid value of the resin constituting the resin particles is preferably 5 mgKOH/g or more and 100 mgKOH/g or less. The weight average molecular weight of the water-soluble resin is preferably 3,000 or more and 15,000 or less. The weight-average molecular weight of the resin constituting the resin particles is preferably 1,000 or more and 2,000,000 or less. The average particle size (volume-based cumulative 50% particle size (D ₅₀ )) of the resin particles measured by a dynamic light scattering method is preferably 100 nm or more and 500 nm or less.

Examples of resins include acrylic resins, urethane resins, and olefin resins. Among them, acrylic resins and urethane resins are preferable.

As the acrylic resin, one having a hydrophilic unit and a hydrophobic unit as structural units is preferable. Among them, a resin having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one of a monomer having an aromatic ring and a (meth)acrylic acid ester-based monomer is preferable. In particular, a resin having a hydrophilic unit derived from (meth)acrylic acid and a hydrophobic unit derived from at least one monomer of styrene and α-methylstyrene is preferred. Since these resins easily interact with pigments, they can be suitably used as resin dispersants for dispersing pigments.

A hydrophilic unit is a unit having a hydrophilic group such as an anionic group. A hydrophilic unit can be formed, for example, by polymerizing a hydrophilic monomer having a hydrophilic group. Specific examples of hydrophilic monomers having hydrophilic groups include acidic monomers having carboxylic acid groups such as (meth)acrylic acid, itaconic acid, maleic acid and fumaric acid, and anions such as anhydrides and salts of these acidic monomers. organic monomers, and the like. Examples of cations constituting salts of acidic monomers include ions such as lithium, sodium, potassium, ammonium and organic ammonium. A hydrophobic unit is a unit that does not have a hydrophilic group such as an anionic group. A hydrophobic unit can be formed, for example, by polymerizing a hydrophobic monomer that does not have a hydrophilic group such as an anionic group. Specific examples of hydrophobic monomers include monomers having an aromatic ring such as styrene, α-methylstyrene, and benzyl (meth)acrylate; methyl (meth)acrylate, butyl (meth)acrylate, (meth)acrylic acid 2 - (Meth)acrylic acid ester monomers such as ethylhexyl. Furthermore, a monomer having an alkylene oxide group may be used.

A urethane-based resin can be obtained, for example, by reacting a polyisocyanate with a polyol. Moreover, what made the chain extension agent further react may be used. Examples of olefinic resins include polyethylene and polypropylene.

[Resin Particle Having Unit Having Alkylene Oxide Group]
In the present invention, the difference η _{2 −η 1 ( viscosity difference η 2 −} η ₁ ) is 4.0 mPa·s or more. Using only a polyhydric alcohol compound that is solid at 25° C. (hereinafter also simply referred to as a “polyhydric alcohol compound”), the composition of the ink is adjusted so that the viscosity of the evaporated ink can reduce negative curl. If you try to adjust it, the amount of polyhydric alcohol compound will increase. Therefore, if an attempt is made to increase the viscosity difference η ₂ -η ₁ , η ₁ also becomes too high, making it difficult to sufficiently suppress smear. In other words, if you try to adjust the viscosity of the evaporated ink using only a polyhydric alcohol compound, there is likely to be a trade-off relationship between negative curl and smear. It may become difficult.

On the other hand, it is difficult to increase the viscosity difference η ₂ −η ₁ for inks in which the viscosity η ₁ is adjusted only with a polyhydric alcohol compound in order to suppress the occurrence of smear, and the effect of suppressing negative curl is slightly reduced. There is In order to achieve both the effect of suppressing negative curl and the effect of suppressing smear at a high level, it is preferable to lower the ink viscosity η ₁ and increase the viscosity difference η ₂ -η ₁ . As a result of investigation, the present inventors found that the ink contains resin particles containing a unit having an alkylene oxide group, in addition to the polyhydric alcohol compound. By using resin particles containing a unit having an alkylene oxide group, the viscosity η ₁ of the ink can be lowered and the viscosity difference η ₂ −η ₁ can be increased as compared with the case where only a polyhydric alcohol compound is used. be able to.

A resin particle containing a unit having an alkylene oxide group is more likely to reduce the ink viscosity η ₁ than the same amount of polyhydric alcohol compound. It is considered that this is because, due to the particle shape, steric interaction between molecules is less than that of polyhydric alcohol compounds, water-soluble resins, and the like, which are solid at 25°C. On the other hand, as the water in the ink evaporates, the polyhydric alcohol compound and the resin particles tend to interact with each other. However, it is believed that the viscosity difference η ₂ −η ₁ can be increased because the hydroxy group of the polyhydric alcohol compound interacts with the alkylene oxide group of the resin particles through hydrogen bonding.

The resin particles containing units having an alkylene oxide group preferably further contain a hydrophilic unit and a hydrophobic unit as described above as suitable units constituting the acrylic resin. The ratio (% by mass) of units having an alkylene oxide group in the resin particles is preferably 3.0% by mass or more. This makes it possible to efficiently increase the viscosity difference η ₂ -η ₁ and further reduce the occurrence of negative curl. The ratio (% by mass) of units having an alkylene oxide group in the resin particles is preferably 10.0% by mass or less, more preferably 5.0% by mass or less.

[Monomer Having Alkylene Oxide Group]
Examples of the alkylene oxide group include an ethylene oxide group and a propylene oxide group. A unit having an alkylene oxide group can be formed by polymerizing a monomer having an alkylene oxide group. Examples of monomers having an alkylene oxide group include (poly)alkylene glycol (meth)acrylates such as ethylene glycol (meth)acrylate, triethylene glycol (meth)acrylate, and propylene glycol (meth)acrylate; methoxyethylene glycol (meth) Alkoxy (poly) alkylene glycol (meth) acrylates such as acrylate, methoxytriethylene glycol (meth) acrylate, ethoxyethylene glycol (meth) acrylate, methoxypropylene glycol (meth) acrylate; phenoxyethylene glycol (meth) acrylate, phenoxy Examples include aryloxy (poly)alkylene glycol (meth)acrylates such as triethylene glycol (meth)acrylate and phenoxypropylene glycol (meth)acrylate. Moreover, a reactive surfactant having an alkylene oxide group can also be used as the monomer having an alkylene oxide group.

A unit having an alkylene oxide group is preferably a unit derived from a monomer having 8 or more alkylene oxide groups. That is, the number of alkylene oxide groups in the monomer having alkylene oxide groups is preferably 8 or more. By using resin particles containing units derived from a monomer having 8 or more alkylene oxide groups, it is possible to further increase the viscosity difference η ₂ -η ₁ and further reduce the occurrence of negative curl. The number of alkylene oxide groups in the monomer having an alkylene oxide group is preferably 15 or less, more preferably 12 or less. Here, "the number of alkylene oxide groups" in the monomer having an alkylene oxide group means "total number of ethylene oxide groups and propylene oxide groups" when, for example, ethylene oxide groups and propylene oxide groups are used. .

The surface charge amount (μmol/g) of the resin particles containing units having an alkylene oxide group is preferably 500 μmol/g or less. If the surface charge amount exceeds 500 μmol/g, the amount of ions present in the ink may increase due to the resin particles. For this reason, the evaporation of water tends to salt out the pigment (especially the pigment dispersed by the action of the anionic group), and the viscosity difference η ₂ - η ₁ becomes too high. may be lower. The surface charge amount of the resin particles is preferably 1 μmol/g or more, more preferably 100 μmol/g or more.

The surface charge amount of resin particles can be measured by colloidal titration. Specifically, first, resin particles separated from ink or the like by an appropriate method are added to water, adjusted to pH 2 with a mineral acid such as hydrochloric acid, and stirred for 24 hours. Thereafter, the resin precipitated by centrifugation is collected and dried. 30 g of a 0.1 mol/L sodium hydrogen carbonate aqueous solution is added to 1 g of the pulverized resin, and the mixture is stirred for 15 hours to obtain a liquid containing the resin. 1 g of the obtained liquid is diluted by adding pure water to obtain 15 g of the diluted liquid. The resin particles in the diluted solution are subjected to colloidal titration using a potential difference to obtain the charge amount per unit mass of the resin particles. Then, the surface charge amount of the resin particles can be calculated by dividing the obtained charge amount by the surface area of the resin particles calculated from the volume-based cumulative 50% particle diameter (D ₅₀ ). Titration uses a titration device such as a potentiometric automatic titrator (trade name “AT510”, manufactured by Kyoto Electronics Industry) equipped with a streaming potential titration unit (PCD-500), and a titration reagent such as 0.1 mol / L hydrochloric acid is used. can be performed using Of course, the titration device to be used and the measurement conditions are not limited to those described above.

[Aqueous medium]
The ink used in the recording method of the present invention is an aqueous ink containing at least water as an aqueous medium. The ink can contain water or an aqueous medium that is a mixed solvent of water and a water-soluble organic solvent. As water, it is preferable to use deionized water or ion-exchanged water. The water content (% by mass) in the water-based ink is preferably 30.0% by mass or more and 95.0% by mass or less based on the total mass of the ink. As described above, among the liquid components constituting the water-based ink for inkjet, water is the most likely to swell the recording medium. Therefore, by reducing the amount of water in the ink, the occurrence of negative curl can be more effectively suppressed. Therefore, the content (% by mass) of water in the water-based ink is preferably 66.0% by mass or less, more preferably 35.0% by mass or more and 65.0% by mass or less, based on the total mass of the ink. is more preferred.

The content (% by mass) of the water-soluble organic solvent in the ink is preferably 3.0% by mass or more and 50.0% by mass or less based on the total mass of the ink. This "water-soluble organic solvent content" is a value including a polyhydric alcohol compound that is solid at 25°C, which will be described later. As the water-soluble organic solvent, alcohols, (poly)alkylene glycols, glycol ethers, nitrogen-containing compounds, sulfur-containing compounds, and the like, which can be used for inkjet inks, can be used.

Normally, the term "water-soluble organic solvent" means a liquid, but in the present invention, a water-soluble organic solvent that is solid at 25°C (normal temperature) is also included in the water-soluble organic solvent. Specific examples of water-soluble organic solvents that are commonly used for water-based inks and are solid at 25° C. include 1,6-hexanediol, trimethylolpropane, ethylene urea, urea, and polyethylene glycol having a number average molecular weight of 1,000. can be mentioned. Some of these are classified as polyhydric alcohol compounds that are solid at 25°C.

[Polyhydric alcohol compound that is solid at 25°C]
As described above, the occurrence of negative curl can be suppressed by increasing the viscosity of the evaporated ink, but if the viscosity of the evaporated ink is too high, smear tends to occur. The viscosity of vaporized ink can be increased in various ways. For example, when a pigment that is dispersed by the action of an anionic group is used, the viscosity of the evaporated ink can be adjusted by controlling the types and contents of the following components. Specifically, water-soluble metal salts such as calcium salts; water-soluble organic solvents such as poor solvents; resins such as water-soluble resins and resin particles; polyhydric alcohol compounds that are solid at 25°C; can.

Among them, it was found that it is preferable to use a polyhydric alcohol compound that is solid at 25°C. When metal salts or resins (especially water-soluble resins) are used, even if the amount is small, the viscosity of the evaporated ink increases significantly, and the ink tends to remain on the surface of the recording medium, tending to cause smearing. . On the other hand, if the content of the metal salt or resin is reduced to a level at which smearing is suppressed, it becomes difficult to increase the viscosity of the vaporized ink. In the case of compounds such as urea and betaine compounds, it is also difficult to increase the viscosity of vaporized ink.

In contrast, by adding an appropriate amount of a polyhydric alcohol compound that is solid at 25° C. to the ink, the occurrence of smear can be more effectively suppressed. A polyhydric alcohol compound that is solid at 25° C. has high crystallinity, and multiple molecules tend to interact with each other when water evaporates. Furthermore, since a polyhydric alcohol compound that is solid at 25° C. has a plurality of hydroxy groups in one molecule, interactions due to hydrogen bonding also occur. These interactions can increase the viscosity of the vaporized ink within an effective range for suppressing negative curling. The content (% by mass) of the polyhydric alcohol compound in the ink is preferably 10.0% by mass or more, more preferably 35.0% by mass or less, based on the total mass of the ink. It is particularly preferably 0% by mass or less.

The polyhydric alcohol compound that is solid at 25° C. is not limited as long as it satisfies this condition. Compounds having a main chain (hydrocarbon chain) of 1 to 6 carbon atoms are particularly preferred. Specific examples include 1,6-hexanediol, trimethylolpropane, trimethylolethane, and sorbitol. Among them, trimethylolpropane is preferred.

The content (% by mass) of resin particles containing a unit having an alkylene oxide group in the ink is 0.25 times or more as a mass ratio with respect to the content (% by mass) of the polyhydric alcohol compound that is solid at 25°C. It is preferably 1.00 times or less. If the mass ratio is _{less than} 0.25 _times , the amount of the polyhydric alcohol compound is greater than that of the resin particles. It may become a little difficult to suppress smear. On the other hand, if the mass ratio is more than 1.00 times, the polyhydric alcohol compound is less than the resin particles, so it becomes difficult to sufficiently increase the viscosity difference η ₂ -η ₁ , resulting in negative curl. It can be somewhat difficult to control.

[Other additives]
In addition to the above ingredients, various other agents such as surfactants, antifoaming agents, pH adjusters, viscosity adjusters, rust inhibitors, antiseptics, anti-mold agents, antioxidants, and anti-reduction agents may be added to the ink. may contain additives.

[Physical properties of ink]
The viscosity η ₁ (mPa·s) of the ink is 5.0 mPa·s or more, preferably 15.0 mPa·s or less, and more preferably 10.0 mPa·s or less. The viscosity η ₂ (mPa·s) of the evaporated ink obtained by evaporating 20 mass % of the ink is preferably 9.0 mPa·s or more, more preferably 10.0 mPa·s or more. Also, the viscosity η ₂ (mPa·s) of the evaporated ink is preferably 25.0 mPa·s or less, more preferably 20.0 mPa·s or less.

The difference η ₂ −η ₁ between the viscosity η ₂ (mPa·s) of the evaporated ink obtained by evaporating 20% by mass of the ink and the viscosity η ₁ (mPa·s) of the ink is 4.0 mPa·s or more, It is preferably 15.0 mPa·s or less, more preferably 10.0 mPa·s or less. The viscosity of ink and evaporated ink can be controlled by adjusting the types and contents of polyhydric alcohol compounds, water-soluble organic solvents, and resins that are solid at 25°C.

In general, water-based ink for inkjet is stored in a container such as an ink cartridge or an ink bottle, or the container is further placed in a sealable bag in order to suppress evaporation of the ink. be. Therefore, since the viscosity of the ink taken out from these containers is the same as that at the time of preparation, the viscosity of this ink is _η1 , and the viscosity of the ink obtained by evaporating 20% by mass from the taken out ink is _η2 .

The ink used in the recording method of the present invention is a water-based ink ejected from an ink jet recording head. Therefore, from the viewpoint of reliability, it is preferable to appropriately control the physical properties of the ink. Specifically, the surface tension of the ink at 25° C. is preferably 20 mN/m or more and 60 mN/m or less. Further, the pH of the ink at 25° C. is preferably 7.0 or more and 9.5 or less, more preferably 8.0 or more and 9.5 or less.

EXAMPLES The present invention will be described in more detail below with reference to examples, comparative examples, and reference examples, but the present invention is not limited by the following examples as long as it does not exceed the scope of the invention. "Parts" and "%" regarding component amounts are based on mass unless otherwise specified.

<Conditions for measuring physical properties>
(Surface charge amount)
Hydrochloric acid was added to the liquid containing the material to be measured to adjust the pH to 2 and stirred for 24 hours. After centrifuging and precipitating the solid matter, it was dried and pulverized to prepare a sample. After adding 30 g of 0.1 mol/L sodium hydrogencarbonate aqueous solution to 1 g of the prepared sample and stirring for 15 hours, it centrifuged and the supernatant was fractionated. A liquid diluted to 15 g by adding pure water to 1 g of the supernatant is titrated with 0.1 mol / L hydrochloric acid using a potentiometric automatic titrator (trade name "AT510", manufactured by Kyoto Electronics Industry), and the charge of the resin particles amount was measured. The surface charge amount of the resin particles was calculated by dividing the measured charge amount by the surface area of the resin particles calculated from the volume-based cumulative 50% particle diameter (D ₅₀ ) measured by the method described later.

(Volume-based cumulative 50% particle size (D ₅₀ ))
A sample having a solid content of about 1.0% was prepared by adding deionized water to a liquid containing the material to be measured. For the prepared sample, using a particle size analyzer (trade name "UPA-EX150", manufactured by Nikkiso) using the dynamic light scattering method, the 50% particle diameter (D ₅₀ ) was measured. A resin whose particle size was not measured by this method is judged to be a "water-soluble" resin.
[Measurement condition]
SetZero: 30 seconds Number of measurements: 3 Measurement time: 180 seconds Refractive index: 1.5

<Preparation of pigment dispersion>
(Pigment dispersions 1 to 4)
A mixture of 25.0 parts of a pigment, 25.0 parts of an aqueous resin solution, and 50.0 parts of pure water is placed in a batch-type vertical sand mill (manufactured by Imex) filled with 200 parts of zirconia beads having a diameter of 0.3 mm. The mixture was dispersed for 5 hours while cooling with water. As a pigment, C.I. I. Pigment Red 122 was used. As the aqueous solution of the resin, a styrene-ethyl acrylate-acrylic acid copolymer having an acid value of 150 mgKOH/g and a weight average molecular weight of 8,000 was dissolved in water containing potassium hydroxide in an equimolar amount to the acid value. An aqueous solution with a resin (solid content) content of 20.0% was used. A pigment dispersion liquid 1 was obtained by pressure filtration through a cellulose acetate filter (manufactured by Advantech) having a pore size of 3.0 μm. The pigment content in pigment dispersion 1 was 25.0%, and the resin content was 5.0%.

The type of pigment is C.I. I. Pigment Blue 15:3, C.I. I. pigment yellow 74 and carbon black. Except for this, Pigment Dispersions 2 to 4 having a pigment content of 25.0% and a resin content of 5.0% were prepared in the same manner as in Pigment Dispersion 1 described above.

(Pigment dispersion liquid 5)
A solution of 5.0 g of concentrated hydrochloric acid dissolved in 5.5 g of water was cooled to 5° C., and 1.1 g of 4-aminophthalic acid was added in this state. The container containing this solution was placed in an ice bath, and while stirring to keep the temperature of the solution below 10°C, a solution obtained by dissolving 1.8 g of sodium nitrite in 9.0 g of deionized water at 5°C was prepared. added. After stirring for 15 minutes, 6.0 g of carbon black (specific surface area: 220 m ² /g, DBP oil absorption: 105 mL/100 g) was added with stirring, and the mixture was further stirred for 15 minutes to obtain a slurry. The resulting slurry was filtered through a filter paper (trade name “Standard Filter Paper No. 2”, manufactured by Advantech), and the particles were thoroughly washed with water and dried in an oven at 110°C. Thereafter, sodium ions were replaced with potassium ions by an ion exchange method to obtain a self-dispersing pigment in which —C ₆ H ₃ —(COOK) ₂ groups were bonded to the carbon black particle surfaces. An appropriate amount of water was added to adjust the pigment content, and a pigment dispersion liquid 5 having a pigment content of 25.0% was obtained.

(Pigment dispersion liquid 6)
A “copper phthalocyanine pigment-containing polymer fine particle dispersion” was prepared according to the description of Preparation Example 1 of Patent Document 2, and this was used as Pigment Dispersion Liquid 6 . The pigment content in the pigment dispersion 6 was 13.0%, and the resin content was 7.0%.

<Synthesis of Resin>
78.8 parts of ion-exchanged water and 0.2 parts of potassium persulfate (polymerization initiator) were placed in a four-necked flask equipped with a stirrer, a reflux condenser, and a nitrogen gas inlet tube, and nitrogen gas was introduced. The mixture was stirred and heated to 80°C. Into this flask was added dropwise a mixture of monomers of types and amounts (unit: parts) shown in the upper part of Tables 1-1 and 1-2 over 2 hours. After aging for 2 hours, it was cooled to 25°C. Appropriate amounts of potassium hydroxide and ion-exchanged water were added to adjust the pH to 8.5 to obtain an aqueous dispersion of resin particles having a resin (solid content) content of 30.0%. The physical properties of the resin particles are shown in the lower part of Tables 1-1 and 1-2. Surfactants 1-6 are compounds having the structures shown below. In the formula below, R represents an alkylene oxide group, and n represents the number of R (alkylene oxide groups). When n=0, R represents a single bond.

(Aqueous solution of resin 1)
After putting 100.0 parts of ethylene glycol monobutyl ether into a four-necked flask equipped with a stirrer, a reflux condenser and a nitrogen gas introduction tube, nitrogen gas was introduced and the temperature was raised to 110° C. with stirring. A mixture of 17.0 parts of acrylic acid, 20.0 parts of n-butyl acrylate, and 63.0 parts of styrene, and 1.3 parts of t-butyl peroxide (polymerization initiator) of ethylene glycol monobutyl ether were added to the flask. The solution was added dropwise over 3 hours. After aging for 2 hours, the ethylene glycol monobutyl ether was removed under reduced pressure to give a solid resin. Potassium hydroxide in an amount equivalent to the acid value and an appropriate amount of ion-exchanged water were added to the obtained resin. The resin was dissolved by heating to 80° C. to obtain an aqueous solution of resin 1 with a resin (solid content) content of 15.0%. Resin 1 was a water-soluble resin.

<Ink preparation>
Each component (unit: %) shown in the upper part of Tables 2-1 to 2-4 was mixed, thoroughly stirred, and then pressurized with a membrane filter with a pore size of 1.2 μm (trade name “HDCII filter”, manufactured by Pall). Each ink was prepared by filtration. In Tables 2-1 to 2-4, the numbers attached to polyethylene glycol are number average molecular weights, and "Acetylenol E60" is a trade name of a surfactant manufactured by Kawaken Fine Chemicals. Ink properties are shown in the lower part of Tables 2-1 to 2-4. The viscosity was measured at 25° C. using an E-type viscometer (manufactured by Toki Sangyo).

<Evaluation>
Using the inkjet recording apparatus 1 having the configuration shown in FIG. 1 and each ink shown in Table 3, the following evaluations were performed. 2A to 2C are used in the case of reverse side ejection, and the transport paths shown in FIGS. 3A to 3D are used in the case of front side ejection. The linear velocity was set to 10 inches/second, and the time from application of ink to the trailing edge portion of the recording medium to paper discharge was set to 2 seconds. In Table 3, "F/D" means "back side discharge" and "F/U" means "front side discharge".

Solid image of 19 cm x 26 cm under the condition of applying 10 ng of ink to a unit area of 1/600 inch x 1/600 inch of A4 size recording medium (plain paper, product name “SW-101”, manufactured by Canon). was recorded continuously for 10 sheets. The recording conditions were an environment of 25° C. and a relative humidity of 50%. In the present invention, in the following evaluation criteria for each item, "AA", "A", and "B" were defined as acceptable levels, and "C" and "D" were defined as unacceptable levels. Table 3 shows evaluation conditions and evaluation results.

(minus curl)
Immediately after 10 sheets of recorded matter were discharged, the state of minus curl was checked, and the minus curl was evaluated according to the evaluation criteria shown below.
AA: No minus curl occurred in any of the printed matter.
A: Immediately after the paper was ejected, there was a recorded material that was instantaneously negatively curled.
B: Minus curl occurred in at least part of the recorded matter, but no jam occurred during paper ejection.
C: Minus curl occurred in at least part of the recorded matter, and jam occurred 1 to 5 times during paper ejection, but the recorded matter did not become cylindrical.
D: Significant negative curl to the extent that the printed matter became cylindrical occurred in at least a part of the printed matter, and jam occurred 1 to 5 times during paper discharge.

(smear)
Immediately after the 10 sheets of recorded matter were discharged, the state of smear on the back side of the 9th recorded matter was checked, and the smear was evaluated according to the following evaluation criteria.
AA: No smear occurred.
A: 10 or less linear smears were observed.
B: More than 10 linear smears were observed, but no solid smears were observed.
C: A solid smear was observed.

1: Inkjet recording device 2: Recording unit 5A: First cassette 5B: Second cassette 7: Conveying roller 8: Recording head 13: Discharge tray 18: Guide S: Recording medium P: Recording area

Claims (15)

An inkjet recording method comprising a recording step of recording an image on a recording medium having a first side and a second side by ejecting a water-based ink containing a water-soluble organic solvent from a recording head,
After the recording step, the method further includes a paper ejection step of ejecting the paper with the later recorded surface of the first surface and the second surface facing downward in the direction of gravity,
The viscosity η ₁ (mPa·s) of the water-based ink is 5.0 mPa·s or more and 15.0 mPa·s or less , and the viscosity η ₂ (mPa·s) of the evaporated ink obtained by evaporating 20% by mass of the water-based ink ) and the viscosity η ₁ (mPa s) of the aqueous ink, the difference η ₂ - η ₁ is 4.0 mPa s or more and 15.0 mPa s or less ,
the water-based ink contains a polyhydric alcohol compound that is solid at 25° C.,
An inkjet recording method , wherein the content (% by mass) of the polyhydric alcohol compound in the water-based ink is 10.0% by mass or more based on the total mass of the ink. Viscosity η of the water-based ink ₁ (mPa s) is 5.0 mPa s or more and 14.0 mPa s or less,
Viscosity η of the evaporated ink obtained by evaporating 20% by mass of the water-based ink ₂ (mPa s) and the viscosity η of the water-based ink ₁ (mPa・s) difference η ₂ -η ₁ is 4.0 mPa·s or more and 11.0 mPa·s or less. 3. The inkjet recording method according to claim 1, wherein the content (% by mass) of water in the water-based ink is 66.0% by mass or less based on the total mass of the ink. The inkjet according to any one of claims 1 to 3, wherein the content (% by mass) of water in the water-based ink is 35.0% by mass or more and 65.0% by mass or less based on the total mass of the ink. Recording method. The inkjet recording method according to any one of claims 1 to 4, wherein the viscosity η2 ₍ mPa·s) of the evaporated ink is 9.0 mPa·s or more and 25.0 mPa·s or less. The inkjet recording method according to any one of claims 1 to 5, wherein the content (% by mass) of the polyhydric alcohol compound in the water-based ink is 35.0% by mass or less based on the total mass of the ink. . The water-based ink contains resin particles containing a unit having an alkylene oxide group,
7. The inkjet recording method according to any one of claims 1 to 6, wherein the ratio (% by mass) of the unit having the alkylene oxide group in the resin particles is 3.0% by mass or more. 8. The inkjet recording method according to claim 7 , wherein the proportion (% by mass) of the unit having the alkylene oxide group in the resin particles is 10.0% by mass or less. 9. The inkjet recording method according to claim 7 , wherein the unit having an alkylene oxide group is a unit derived from a monomer having 8 or more alkylene oxide groups. 10. The inkjet recording method according to any one of claims 7 to 9 , wherein the resin particles have a surface charge amount ([mu]mol/g) of 500 [mu]mol/g or less. 8. The content (mass %) of the resin particles in the water - based ink is 0.25 times or more and 1.00 times or less as a mass ratio with respect to the content (mass %) of the polyhydric alcohol compound. 11. The inkjet recording method according to any one of items 1 to 10 . 12. The inkjet recording method according to any one of claims 1 to 11 , wherein the recording medium is a paper-based recording medium having no coat layer. The inkjet recording method according to any one of claims 1 to 12 , wherein the recording head is a line head. An inkjet recording apparatus used for recording an image on a recording medium having a first side and a second side by ejecting water-based ink containing a water-soluble organic solvent from a recording head,
a back side paper ejection mechanism for ejecting paper with the later recorded side of the first side and the second side facing downward in the direction of gravity;
The viscosity η ₁ (mPa·s) of the water-based ink is 5.0 mPa·s or more and 15.0 mPa·s or less , and the viscosity η ₂ (mPa·s) of the evaporated ink obtained by evaporating 20% by mass of the water-based ink ) and the viscosity η ₁ (mPa s) of the aqueous ink, the difference η ₂ - η ₁ is 4.0 mPa s or more and 15.0 mPa s or less ,
the water-based ink contains a polyhydric alcohol compound that is solid at 25° C.,
An inkjet recording apparatus , wherein the content (% by mass) of the polyhydric alcohol compound in the water-based ink is 10.0% by mass or more based on the total mass of the ink. Viscosity η of the water-based ink ₁ (mPa s) is 5.0 mPa s or more and 14.0 mPa s or less,
Viscosity η of the evaporated ink obtained by evaporating 20% by mass of the water-based ink ₂ (mPa s) and the viscosity η of the water-based ink ₁ (mPa・s) difference η ₂ -η ₁ is 4.0 mPa·s or more and 11.0 mPa·s or less.

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