Inkjet printing method

Abstract

To provide an inkjet printing method which can suppress printing irregularities and is excellent in solid filling properties even when performing the printing to liquid non-absorbing printing medium.SOLUTION: An inkjet printing method using two colors or more is the inkjet printing method including a process of forming an image by supplying inks of at least two colors to a printing medium in such conditions that the ratio of droplet separability, of ink of color discharged at the first time is larger than the ratio of droplet separability, of ink of color discharged at the second time or later.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an inkjet printing method. [Background technology]

[0002] Printing using the inkjet recording method has many advantages, such as no need for plate making, ease of handling variable information, and no contact with the print medium when printing on the print medium. Therefore, in addition to being used in office and home applications, in recent years it has also been expanded to commercial printing applications where a much larger number of pages are to be printed. For commercial printing applications, in addition to low-absorbency print media such as printing paper, there is a demand for support for non-absorbency print media such as resin films. In response to such demands, methods have been proposed that enable image formation to be completed quickly even on non-liquid-absorbent printing media.

[0003] For example, in Patent Document 1, for the purpose of providing an inkjet recording method capable of high-speed printing with high image quality without reducing the amount of ink applied and without causing printing unevenness, a one-pass type inkjet recording method using water-based ink is described, in which water-based ink having a surface tension of 35 mN / m or less is used, and the absorption rate of the water-based ink is 0.05 ml / m 2 ms 1 / 2 More than 1ml / m 2 ms 1 / 2 The document describes an inkjet recording method in which, when an image is formed by printing on the following coated printing paper, drying begins within 0.5 seconds after the start of printing. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-285926 A Summary of the Invention [Problem to be solved by the invention]

[0005] Conventionally, it has been considered difficult to apply printing by inkjet recording to mass printing, but by improving ink and devising drying methods, it has been possible to improve printing speed and accommodate printing media of various materials. However, in printing by inkjet recording on non-absorbent printing media such as resin films, for non-absorbent printing media in which the residual liquid components of the inkjet ink are difficult to eliminate, there is a problem that the coalescence of ink dots cannot be suppressed in some areas where the ink is not sufficiently dried, resulting in uneven printing. In addition, in printing by inkjet recording on non-absorbent printing media, there is a problem that the ink is poorly wetted and spreadable on the printing medium, resulting in a decrease in printing quality such as solid filling. However, the technology of Patent Document 1 is not fully satisfactory for such problems, and there is room for improvement. Thus, there has been a demand for the development of a technology that can suppress uneven printing and has excellent solid filling even in printing by inkjet recording using non-absorbent printing media, specifically resin films. An object of the present invention is to provide an inkjet printing method that can suppress print unevenness in the resulting print even when printing on a non-liquid-absorbent print medium, and that has excellent solid filling properties. [Means for solving the problem]

[0006] The inventors have discovered that the above-mentioned problems can be solved by including a step of applying two or more colors of ink to a printing medium to form an image, under conditions where the droplet rate of the ink ejected in the first color is greater than the droplet rate of the ink ejected in the second or subsequent colors, in ink jet printing using two or more colors. In other words, the present invention provides an inkjet printing method using ink of two or more colors, which includes a step of applying ink of two or more colors to a printing medium to form an image under conditions where the droplet rate of the ink ejected in the first color is greater than the droplet rate of the ink ejected in the second or subsequent colors. Effect of the Invention

[0007] According to the present invention, it is possible to provide an inkjet printing method that can suppress printing unevenness in the resulting print even when printing on a non-liquid-absorbent print medium, and that has excellent solid filling properties. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] [Inkjet printing method] The inkjet printing method of the present invention (hereinafter also referred to as "the printing method of the present invention") is an inkjet printing method that uses ink of two or more colors, and includes a step of applying ink of two or more colors to a printing medium to form an image under conditions where the droplet rate of the ink ejected in the first color is greater than the droplet rate of the ink ejected in the second color and thereafter.

[0009] In the present invention, "the ejected ink is divided into droplets" means that the ink ejected based on one ejection command to eject ink from the inkjet head is separated into two or more droplets and applied to the printing medium. In the present invention, the "droplet rate" is calculated from the following formula as the ratio of the number of nozzles in which droplets were confirmed to be ejected out of 20 specific nozzles using an inkjet ejection observation device (a device which adjusts the interval of strobe light emission so that the ink ejected from the inkjet head appears to be stationary, and which captures the ink that appears to be stationary with a video camera (frame rate: 24 fps to 60 fps)). Droplet rate (%) = (number of nozzles with confirmed droplets / 20) x 100 In the present invention, the term "non-liquid absorbing" refers to a printing medium having a water absorption amount of 0 g / m2 when the printing medium is in contact with pure water for a contact time of 100 ms. 2 More than 10g / m 2 The water absorption amount can be measured using an automatic scanning absorptivity meter (for example, KM500win manufactured by Kumagai Riki Kogyo Co., Ltd.) under conditions of 23° C. and a relative humidity of 50%, and is defined as the transferred amount of water in a contact time of 100 ms with pure water.

[0010] According to the present invention, even when printing on a non-liquid-absorbent print medium, it is possible to suppress print unevenness in the resulting print and to achieve excellent solid filling. The reason for this is not entirely clear, but is thought to be as follows. Generally, secondary or higher color printing unevenness tends to occur when a coating is formed with two or more colors of ink mixed unevenly. This is because if the volume of each ink droplet applied to the printing medium of the first color is large, the surface area of ​​the ink droplet is small and it takes time to dry on the printing medium after application, so when the ink of the second or subsequent colors is applied to the printing medium, part of each color of ink mixes with each other. In the present invention, by including a step of applying two or more colors of ink to a printing medium to form an image under the condition that the droplet rate of the ink ejected in the first color is greater than the droplet rate of the ink ejected in the second and subsequent colors, the volume of each ink droplet applied to the printing medium in the first color is small, so that the ink of the first color dries quickly on the printing medium to which it is applied. Therefore, when the inks of the second and subsequent colors are applied to the printing medium, the ink of the first color has already formed a coating film on the printing medium, and mixing of the inks of each color can be suppressed, which is thought to result in the formation of a good image with suppressed printing unevenness. In addition, if the ink ejected in the first color is split into droplets, the number of ink droplets applied to the printing medium increases, and the surface of the printing medium can be well covered even on non-absorbent printing media that have poor wetting and spreading properties. Once a coating film is formed with the first color ink, the surface of the non-absorbent printing medium is covered, so the inks of the second and subsequent colors have good wetting and spreading properties, and it is believed that even when printing on a non-absorbent printing medium, it is possible to obtain a printed matter with excellent print quality due to improved solid filling.

[0011] (image forming process) The printing method of the present invention includes a process of forming an image (hereinafter also referred to as the "image forming process") by applying two or more colors of ink to a printing medium by an inkjet recording method under conditions where the droplet rate of the ink ejected in the first color is greater than the droplet rate of the ink ejected in the second color and thereafter.

[0012] <Print media> Preferred examples of the printing medium used in the printing method of the present invention include coated paper and resin films, and more preferred are resin films. Suitable examples of the resin film include polyester films such as polyethylene terephthalate films; vinyl chloride films; polyolefin films such as polypropylene films and polyethylene films; and polyamide films such as nylon films. These resin films may be stretched films such as biaxially stretched films and uniaxially stretched films, or non-stretched films. Among these, the printing medium is more preferably one or more selected from the group consisting of polyethylene terephthalate films and biaxially stretched polypropylene films, and even more preferably a polyethylene terephthalate film that has been subjected to a surface treatment such as a corona discharge treatment, or a biaxially stretched polypropylene film that has been subjected to the surface treatment.

[0013] <Drop rate> In the printing method of the present invention, inks of two or more colors are applied to a print medium under the condition that the droplet rate of the ink ejected in the first color is greater than the droplet rates of the inks ejected in the second and subsequent colors. The fact that the droplet rate of the ink ejected in the first color is greater than the droplet rate of the ink ejected in the second and subsequent colors means that the ink ejected in the second and subsequent colors is applied to the printing medium in the form of relatively large ink droplets compared to the portion of the ink of the first color applied to the printing medium at a large droplet rate.This makes it possible to efficiently suppress printing unevenness as described above, and also to satisfactorily coat the surface of the printing medium even on non-absorbent printing media that have low wetting and spreading properties, improving solid filling and resulting in a printed product with excellent print quality.

[0014] In the present invention, the two or more ink droplets formed by the splitting can be jetted in a state much closer to each other than when a command to eject ink is executed twice with the interval as short as possible under the condition of not splitting the ink, and the volume of each ink droplet is also small. Therefore, as described above, it is possible to efficiently suppress printing unevenness, and even on a non-absorbent printing medium with low wettability and spreadability, the surface of the printing medium can be well covered, and the solid filling ability is improved, resulting in a printed matter with excellent print quality. From this point of view, it is preferable that the ink ejected based on one ejection command to eject ink from the inkjet head is separated into two or more to three droplets and applied to the printing medium, and more preferably, it is separated into two droplets and applied to the printing medium. In the present invention, the number of ink droplets is counted by the number of ink droplets present within a distance range of 1 mm or more and 2 mm or less from the nozzle plate of the inkjet head when observing the ink ejected based on a single ejection command from the inkjet head using an inkjet ejection observation device (a device that adjusts the interval of strobe emission so that the ink ejected from the inkjet head appears to be stationary, and captures the ink droplets that appear to be stationary with a video camera (frame rate: 24 fps or more and 60 fps or less)), setting conditions (imaging magnification: 180x) such that ink droplets are observed within the distance range.

[0015] In the present invention, as described above, the "droplet rate" is calculated from the following formula as the ratio of the number of nozzles in which separated droplets were confirmed out of the 20 observed nozzles, when ink ejected based on a single ejection command for ejecting ink from the inkjet head is confirmed to be ejected in droplets from 20 specific nozzles using the inkjet ejection observation device described above. Droplet rate (%) = (number of nozzles with confirmed droplets / 20) x 100 In the present invention, the droplet rate of the ink ejected in the first color must be greater than the droplet rate of the ink ejected in the second and subsequent colors, and the droplet rate of the ink in the second and subsequent colors may be 0%.

[0016] In the present invention, the droplet ratio of the ink ejected for the first color is, from the viewpoint of suppressing printing unevenness and improving solid filling, preferably 40% or more, more preferably 50% or more, even more preferably 70% or more, still more preferably 90% or more, still more preferably 99% or more, and still more preferably 100%. In addition, in the present invention, the droplet rate of ink ejected from the second color onwards is, from the viewpoint of suppressing printing unevenness and improving solid filling, preferably 50% or less, more preferably 40% or less, even more preferably 30% or less, even more preferably 20% or less, even more preferably 10% or less, even more preferably 1% or less, and preferably 0% or more, even more preferably 0%. In the printing method of the present invention, from the viewpoint of suppressing printing unevenness and improving solid filling, it is preferable that the droplet rate is controlled by one or more methods selected from the group consisting of a method of adjusting the temperature inside the inkjet head and a method of adjusting the ejection force of the inkjet head.

[0017] In the printing method of the present invention, the droplet rate is preferably controlled by adjusting the temperature inside the inkjet head, from the viewpoints of suppressing printing unevenness and improving solid printing performance. When the temperature inside the inkjet head is high, the temperature of the inkjet ink filling the inkjet head also increases, and the surface tension decreases. This reduces the ink's tendency to gather into a single droplet when ejected, making it easier for droplets to split. The temperature inside the inkjet head is preferably adjusted by a mechanism for forcibly circulating the ink inside the inkjet head. That is, in the printing method of the present invention, it is preferable that the inkjet printing is performed by a circulation type inkjet head. The temperature inside the inkjet head for controlling the droplet rate is, from the viewpoint of suppressing printing unevenness and improving solid filling, preferably 25° C. or higher, more preferably 30° C. or higher, even more preferably 32° C. or higher, still more preferably 35° C. or higher, and is preferably 50° C. or lower, more preferably 49° C. or lower, even more preferably 48° C. or lower, still more preferably 47° C. or lower, and still more preferably 46° C. or lower.

[0018] In the present invention, from the viewpoint of suppressing printing unevenness and improving solid filling, it is preferable to adjust the temperature inside the inkjet head for the first color ink and the second and subsequent colors of ink, and it is preferable that the temperature inside the inkjet head when applying the first color ink is higher than the temperature inside the inkjet head when applying the second and subsequent colors of ink. From the viewpoint of suppressing printing unevenness and improving solid filling, the temperature inside the inkjet head when applying the first color of ink is preferably 38° C. or more, more preferably 40° C. or more, even more preferably 43° C. or more, still more preferably 45° C. or more, and is preferably 50° C. or less, more preferably 49° C. or less, even more preferably 48° C. or less, still more preferably 47° C. or less, and still more preferably 46° C. or less. The temperature inside the inkjet head when applying the second or subsequent colors of ink is, from the viewpoint of suppressing printing unevenness and improving solid filling, preferably 25° C. or more, more preferably 30° C. or more, even more preferably 34° C. or more, still more preferably 35° C. or more, and is preferably 50° C. or less, more preferably 45° C. or less, even more preferably 40° C. or less, and still more preferably 36° C. or less.

[0019] In the printing method of the present invention, the droplet rate is preferably controlled by adjusting the ejection force of the inkjet head, from the viewpoints of suppressing printing unevenness and improving solid printing performance. The ejection force of the inkjet head is the source that gives the ink kinetic energy for flying, and the droplet rate can be controlled by adjusting the ejection force. When the ejection method of the inkjet head is a piezoelectric method, suitable methods for adjusting the ejection force include a method of changing the positive / negative or absolute value of the voltage applied to the piezoelectric element, and a method of changing the rate of increase or decrease of the voltage applied to the piezoelectric element. When the ejection method of the inkjet head is a thermal method, suitable methods for adjusting the ejection force include a method of changing the absolute value of the voltage applied to the heater portion of the inkjet head, and a method of controlling the conditions of a so-called preheat operation in which a voltage that does not result in ejection is applied in advance to prepare for ejection before the voltage at the time of ejection is applied to the heater.

[0020] When the ejection method of the inkjet head is a piezoelectric method, from the viewpoint of suppressing printing unevenness and improving solid filling, it is preferable to adjust the voltage applied to the piezoelectric element of the inkjet head for the first color of ink and the second and subsequent colors of ink. The voltage applied to the piezoelectric element of the inkjet head for controlling the droplet rate when depositing the first color of ink is preferably adjusted so that the ratio of the voltage applied to the piezoelectric element to the standard value is preferably 110% or more, more preferably 115% or more, even more preferably 120% or more, still more preferably 125% or more, even more preferably 130% or more, and preferably 150% or less, more preferably 145% or less, and even more preferably 140% or less. In addition, the voltage applied to the piezoelectric element of the inkjet head for controlling the droplet rate when applying the second or subsequent colors of ink is preferably adjusted so that the ratio of the voltage applied to the piezoelectric element to the standard value is preferably 100% or more, and preferably 120% or less, more preferably 115% or less, even more preferably 110% or less, and even more preferably 105% or less. Here, the standard value of the applied voltage to the piezoelectric element means the voltage recommended in the specifications as the voltage to be applied to the inkjet head used in inkjet printing, and the numerical value stated in the specifications or catalog can be referenced. Specifically, for example, when an inkjet head is used in which the standard value of the applied voltage to the piezoelectric element is 26 V, the voltage applied to the piezoelectric element of the inkjet head to control the droplet rate when depositing the first color of ink is preferably 29 V or more, more preferably 30 V or more, even more preferably 31 V or more, still more preferably 33 V or more, and preferably 39 V or less, more preferably 38 V or less, and even more preferably 36 V or less. In addition, the voltage applied to the piezoelectric element of the inkjet head for controlling the droplet rate when applying the second or subsequent colors of ink is preferably 26 V or more, and preferably 35 V or less, more preferably 30 V or less, and even more preferably 27 V or less.

[0021] When the inkjet head uses a piezoelectric ejection method, the combination of the temperature inside the inkjet head and the voltage applied to the piezoelectric element for controlling the droplet rate when applying the first color of ink is preferably such that the temperature inside the inkjet head is 38°C or higher and the voltage applied to the piezoelectric element is equal to or higher than a voltage corresponding to 110% of the standard value of the voltage applied to the piezoelectric element, more preferably 40°C or higher and a voltage corresponding to 115% of the standard value, even more preferably 43°C or higher and a voltage corresponding to 120% of the standard value, even more preferably 43°C or higher and a voltage corresponding to 125% of the standard value, and even more preferably 45°C or higher and a voltage corresponding to 130% of the standard value. When using an inkjet head in which the standard value of the voltage applied to the piezoelectric element is 26V, the combination of the temperature inside the inkjet head and the voltage applied to the piezoelectric element to control the droplet rate when applying the first color of ink is preferably such that the temperature inside the inkjet head is 38°C or higher and the voltage applied to the piezoelectric element is 29V or higher, more preferably 40°C or higher and 30V or higher, even more preferably 43°C or higher and 31V or higher, even more preferably 43°C or higher and 33V or higher, and even more preferably 45°C or higher and 33V or higher.

[0022] In the present invention, the relationship between the first color ink and the second and subsequent colors is preferably the relationship between the colors printed when forming an image of two or more colors. That is, the present invention is preferably an inkjet printing method using ink of two or more colors, which includes a step of applying ink of two or more colors to a printing medium under the condition that the droplet rate of the ink ejected in the first color is higher than the droplet rate of the ink ejected in the second and subsequent colors to form an image of two or more colors. In this case, the image formed by inkjet printing according to the present invention may include at least a secondary or higher color image forming region by including the image forming step, i.e., the image formed by inkjet printing according to the present invention may be a combination of a primary color image forming region and a secondary or higher color image forming region. Specifically, when forming and printing a blue image as a secondary color using cyan ink and magenta ink, if cyan ink is printed as the first color and magenta ink is printed as the second color, it is preferable that the droplet rate of the cyan ink be greater than the droplet rate of the magenta ink. When forming and printing a red image as a secondary color using magenta ink and yellow ink, if magenta ink is printed as the first color and yellow ink is printed as the second color, it is preferable that the droplet rate of the magenta ink is greater than the droplet rate of the yellow ink. When forming and printing an image of green as a secondary color using cyan ink and yellow ink, if cyan ink is printed as the first color and yellow ink is printed as the second color, it is preferable that the droplet rate of the cyan ink is greater than the droplet rate of the yellow ink. When printing black as a tertiary color using cyan ink, magenta ink, and yellow ink, if cyan ink is printed as the first color, magenta ink is printed as the second color, and yellow ink is printed as the third color, it is preferable that the droplet rate of the cyan ink is greater than at least one of the droplet rates of the magenta ink and the yellow ink. In the present invention, the image formed by the above steps is more preferably a secondary color or tertiary color image, and even more preferably a secondary color image.

[0023] <Inkjet head> As the type of ink-jet head used in the printing method of the present invention, either a line head type or a serial head type can be used. The line head method has an effective length that allows inkjet printing over a width equal to or greater than the length of the print medium in the direction perpendicular to the transport direction, and the inkjet head itself does not move during inkjet printing. The serial head method is a method in which inkjet printing is performed while moving an inkjet head in a direction perpendicular to the transport direction of the print medium. Among these, it is preferable to use a line head type as the inkjet head type from the viewpoint of suppressing printing unevenness and improving solid filling. The line head system may be one in which a plurality of individual heads are arranged in a line. In the present invention, the inkjet head is preferably an inkjet head or a plurality of single heads arranged in a line, one for each ink color.

[0024] From the viewpoints of suppressing printing unevenness and improving solid filling, the nozzle spacing of the inkjet head is preferably 120 npi or more, more preferably 180 npi or more, even more preferably 300 npi or more, and even more preferably 800 npi or more, and from the same viewpoints as above, it is preferably 6000 npi or less, more preferably 3600 npi or less, even more preferably 2400 npi or less, and even more preferably 1600 npi or less. In the present invention, the unit "npi" indicating the nozzle interval of an inkjet head means the number of nozzles per inch in the longitudinal direction of the nozzle row of the inkjet head. In the present invention, when an inkjet head is used alone, the nozzle spacing of the inkjet head itself means the nozzle spacing of the inkjet head described above, and when a plurality of individual heads are combined and arranged so that the nozzles of the individual heads do not overlap each other, it means the nozzle spacing estimated when the nozzles do not overlap. When the nozzle spacing of the inkjet head differs for each ink color, the nozzle spacing of the inkjet head in the present invention is defined as the nozzle spacing for the color having the greatest number of nozzles per inch in the longitudinal direction of the nozzle row of the inkjet head.

[0025] There are various inkjet ejection methods, but in the printing method of the present invention, either a piezoelectric method using a piezoelectric element or a thermal method using a thermal element may be used. Among these, the inkjet ejection method is preferably a piezoelectric method from the viewpoint of controlling the droplet rate, suppressing printing unevenness, and improving solid filling.

[0026] <Printing medium transport speed> In the present invention, the transport speed of the printing medium is preferably 10 m / min or more, more preferably 15 m / min or more, and even more preferably 20 m / min or more, from the viewpoint of efficiently obtaining printed matter, and is preferably 100 m / min or less, more preferably 80 m / min or less, and even more preferably 60 m / min or less, from the viewpoint of suppressing printing unevenness and improving solid filling.

[0027] In the present invention, from the viewpoint of easily controlling the droplet ratio, the amount of ink ejected is preferably 1 pL or more, more preferably 1.5 pL or more, even more preferably 2 pL or more, and is preferably 15 pL or less, more preferably 10 pL or less, even more preferably 7.5 pL or less, and even more preferably 5 pL or less. Here, "pl (picoliter)" is 10 of "l (liter)". -12 means. The ink droplet volume can be set by an inkjet head control device. When the ink droplet is divided into droplets, the ejected ink droplet volume means the total volume of the main ink droplet and the separated ink droplets.

[0028] <Resolution> In the printing method of the present invention, the resolution in a direction parallel to the transport direction of the printing medium, i.e., the density of ink dots on the printing medium, is preferably 600 dpi or more, more preferably 800 dpi or more, even more preferably 1000 dpi or more, from the viewpoints of suppressing printing unevenness and improving solid filling, and from the same viewpoints as above, is preferably 3600 dpi or less, more preferably 2400 dpi or less, even more preferably 1800 dpi or less, and even more preferably 1200 dpi or less. In the printing method of the present invention, the resolution in a direction perpendicular to the transport direction of the printing medium, i.e., the density of ink dots on the printing medium, is preferably 600 dpi or more, more preferably 800 dpi or more, and even more preferably 1000 dpi or more, from the viewpoints of suppressing printing unevenness and improving solid filling, and from the same viewpoints as above, is preferably 3600 dpi or less, more preferably 2400 dpi or less, even more preferably 1800 dpi or less, and even more preferably 1200 dpi or less. In the present invention, the unit of resolution "dpi" means the number of ink dots per inch in a direction parallel or perpendicular to the transport direction of the print medium. When the resolution differs for each ink color, the highest resolution is regarded as the resolution in the present invention.

[0029] <Ink dot diameter> In the printing method of the present invention, the diameter of the ink dots that have landed on the printing medium (hereinafter simply referred to as "ink dot diameter") is preferably 20 μm or more, more preferably 23 μm or more, and even more preferably 25 μm or more, from the viewpoints of suppressing printing unevenness and improving solid filling, and from the same viewpoints as above, it is preferably 50 μm or less, more preferably 45 μm or less, and even more preferably 40 μm or less. In the present invention, the diameter of an ink dot that has landed on a printing medium is considered to be one ink dot if the ink dot lands as a single droplet and coalesces when ejected from an inkjet head. In addition, if the shape of the ink dot is not circular, the average value of the major axis and minor axis is considered to be the diameter of the ink dot that has landed on the printing medium. In addition, in the printing method of the present invention, the ink dots that land on the print medium by inkjet printing have a small diameter, so the formation of an image by inkjet printing is completed quickly, and therefore it can be considered that there is no difference between the diameter of the ink dot immediately after it lands on the print medium by inkjet printing and the diameter of the ink dot after time has passed since it landed on the print medium and the ink has completely dried. As described in the Examples, the diameter of the ink dots that have landed on the printing medium can be determined by observing 10 ink dots under a microscope after applying ink by inkjet printing, and taking the average value as the diameter of the ink dots.

[0030] <Ink> As the ink used in the printing method of the present invention, either an aqueous ink or a non-aqueous ink can be used. From the viewpoints of controlling the droplet rate and efficiently suppressing printing unevenness, and of improving solid filling ability, an aqueous ink is preferred. In the present invention, the term "water-based ink" refers to an ink in which water accounts for the largest proportion by mass of the liquid components of the ink. The water content in the ink used in the printing method of the present invention is preferably 50% by mass or more, more preferably 55% by mass or more, even more preferably 60% by mass or more, and preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less.

[0031] [Coloring material] The ink used in the printing method of the present invention contains a colorant. As the coloring material, dyes, pigments, and a combination of dyes and pigments can be used. From the viewpoint of suppressing printing unevenness and improving solid filling, pigments are preferred. The pigment may be either an inorganic pigment or an organic pigment, and a lake pigment or a fluorescent pigment may also be used. If necessary, these pigments may also be used in combination with an extender pigment. Examples of inorganic pigments include carbon black, metal oxides such as titanium oxide, iron oxide, red iron oxide, and chromium oxide, and pearlescent pigments. Examples of carbon black include furnace black, lamp black, acetylene black, and channel black. Examples of organic pigments include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; and polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and threne pigments. In the achromatic ink, achromatic pigments such as white, black, and gray can be used, while in the chromatic ink, chromatic organic pigments such as cyan, magenta, yellow, blue, red, orange, and green can be used. Preferred organic pigments include, for example, one or more product numbers selected from the group consisting of CI Pigment Blue as a cyan pigment, CI Pigment Red, CI Pigment Orange, and CI Pigment Violet as magenta pigments, and CI Pigment Yellow as a yellow pigment. The pigments may be used singly or in combination of two or more kinds within each of the cyan, magenta and yellow color systems.

[0032] Among these, from the viewpoint of suppressing printing unevenness and improving solid filling, the CI Pigment Blue is preferably one or more selected from the group consisting of CI Pigment Blue 15:3 and CI Pigment Blue 15:4, the CI Pigment Red is preferably one or more selected from the group consisting of CI Pigment Red 122 and CI Pigment Red 150, the CI Pigment Violet is preferably CI Pigment Violet 19, and the CI Pigment Yellow is preferably one or more selected from the group consisting of CI Pigment Yellow 74 and CI Pigment Yellow 155. From the viewpoint of suppressing printing unevenness when forming a secondary color image and from the viewpoint of improving solid filling, a combination of colorants is preferably one or more selected from the group consisting of CI Pigment Blue 15:3 and CI Pigment Blue 15:4 and one or more selected from the group consisting of CI Pigment Red 122, CI Pigment Red 150, and CI Pigment Violet 19 from the viewpoint of forming a blue image as a secondary color, and more preferably one or more selected from the group consisting of CI Pigment Red 122, CI Pigment Red 150, and CI Pigment Violet 19 from the viewpoint of forming a red image as a secondary color. 2. A combination of one or more pigments selected from the group consisting of CI Pigment Red 150 and CI Pigment Violet 19 with one or more pigments selected from the group consisting of CI Pigment Yellow 74 and CI Pigment Yellow 155 is preferred, and from the viewpoint of forming an image with green as a secondary color, a combination of one or more pigments selected from the group consisting of Pigment Blue 15:3 and Pigment Blue 15:4 with one or more pigments selected from the group consisting of CI Pigment Yellow 74 and CI Pigment Yellow 155 is preferred.

[0033] In the present invention, the form of the pigment in the ink may be a form in which the pigment is dispersed using a polymer dispersant or a surfactant as a pigment dispersant, or a form of a self-dispersed pigment dispersed without using a pigment dispersant. The polymer dispersant may be either a water-soluble polymer or a water-insoluble polymer. Here, the "water-soluble" and "water-insoluble" of a polymer are determined as follows: when a polymer is dried at 105°C for 2 hours and allowed to reach a constant weight, and then dissolved in 100 g of water at 25°C until saturation is reached, if the amount dissolved exceeds 10 g, the polymer is determined to be "water-soluble," and if the amount dissolved is 10 g or less, the polymer is determined to be "water-insoluble." In addition, if a polymer has an anionic group and the anionic group is neutralized with a neutralizing agent when the polymer is blended into the ink of the present invention, the "water-soluble" and "water-insoluble" are determined based on the amount of dissolution measured under conditions in which the neutralizing agent is mixed in such that the mass ratio of the polymer to the neutralizing agent is the same when the polymer is blended into the ink of the present invention.

[0034] In the present invention, the form of the pigment in the ink is preferably, specifically, from the viewpoint of improving the dispersion stability of the pigment, controlling the behavior of droplets, suppressing printing unevenness, and improving solid coverage, such as (I) a form in which a water-soluble polymer is adsorbed onto the pigment surface as a polymer dispersant, (II) a form in which a water-soluble surfactant or a water-dispersible surfactant is adsorbed onto the pigment surface, (III) a form of a self-dispersing pigment in which a hydrophilic functional group is chemically or physically introduced onto the pigment surface and dispersed without using a polymer dispersant or a surfactant, and (IV) a form in which the pigment is coated with a water-insoluble polymer as a polymer dispersant. Among these, from the same viewpoint as above, the form of the pigment in the ink in the present invention is more preferably one or more forms selected from the group consisting of form (III) and form (IV), and even more preferably form (IV).

[0035] As the form in which the (IV) pigment is coated with a water-insoluble polymer (hereinafter, the water-insoluble polymer is also referred to as "water-insoluble polymer (p)") as a polymer dispersant, a form of a particle of the water-insoluble polymer (p) containing the pigment (hereinafter, also referred to as "pigment-containing polymer particle" is preferable. Examples of the form of the pigment-containing polymer particle include a form in which the water-insoluble polymer (p) encapsulates the pigment, a form in which the pigment is uniformly dispersed in the water-insoluble polymer (p), a form in which the pigment is exposed from the surface of the water-insoluble polymer (p) particle, a form in which the water-insoluble polymer (p) is adsorbed to the pigment, and a mixture thereof.

[0036] Examples of the water-insoluble polymer (p) include vinyl resins, polyester resins, polyurethane resins, and the like. Among these, vinyl polymers obtained by addition polymerization of vinyl monomers are preferred from the viewpoint of suppressing printing unevenness and improving solid filling. The vinyl polymer preferably contains a structural unit derived from an ionic monomer, and more preferably contains a structural unit derived from a hydrophobic monomer and / or a nonionic monomer. As the ionic monomer, the hydrophobic monomer, and the nonionic monomer, it is preferable to use those exemplified in paragraphs

[0015] to

[0019] of JP 2022-104084 A. Among these, from the viewpoints of improving the dispersion stability of the pigment, controlling the behavior of droplets, suppressing printing unevenness, and improving solid filling ability, it is more preferable that the water-insoluble polymer (p) contains one or more constituent units derived from ionic monomers selected from the group consisting of acrylic acid and methacrylic acid, and one or more constituent units derived from hydrophobic monomers selected from the group consisting of styrene, α-methylstyrene, and benzyl (meth)acrylate.

[0037] The weight average molecular weight of the water-insoluble polymer (p) is preferably 10,000 or more, more preferably 15,000 or more, and preferably 100,000 or less, more preferably 50,000 or less. The weight average molecular weight of the water-insoluble polymer (p) is measured by the method described in the Examples. The acid value of the water-insoluble polymer (p) is preferably 80 mgKOH / g or more, more preferably 120 mgKOH / g or more, even more preferably 160 mgKOH / g or more, and even more preferably 200 mgKOH / g or more, and is preferably 300 mgKOH / g or less, more preferably 280 mgKOH / g or less. The acid value of the water-insoluble polymer (p) can be determined by the method described in the Examples, but can also be calculated from the mass ratio of the constituent monomers.

[0038] The pigment dispersed with a polymer dispersant may be one in which the polymer dispersant is crosslinked with a crosslinking agent, i.e., a pigment dispersed with a polymer dispersant having a crosslinked structure (hereinafter also referred to as a "crosslinked polymer dispersant"), from the viewpoints of improving the dispersion stability of the pigment, controlling the behavior of droplets, suppressing printing unevenness, and improving solid filling ability. When the pigment in the ink according to the present invention is in the form of (IV), it is preferably in the form of particles of a water-insoluble crosslinked polymer containing the pigment (hereinafter also referred to as "pigment-containing crosslinked polymer particles"). The pigment-containing crosslinked polymer particles are composed of a pigment and a crosslinked polymer, and the crosslinked polymer preferably comprises a structure derived from the water-insoluble polymer (p) and a structure derived from the crosslinking agent, and more preferably is obtained by crosslinking the water-insoluble polymer (p) with a crosslinking agent. The crosslinking agent may be a compound having two or more functional groups capable of reacting with the functional groups of the polymer dispersant. For example, when the polymer dispersant has a carboxy group, the crosslinking agent may preferably be a polyglycidyl ether compound of a polyhydric alcohol.

[0039] In the ink used in the printing method of the present invention, when the form of the pigment in the ink is the form (IV), the pigment is preferably blended in the ink as an aqueous dispersion of pigment-containing polymer particles or pigment-containing crosslinked polymer particles (hereinafter also referred to as "pigment dispersion"). The pigment dispersion can be efficiently produced using the pigment and the water-insoluble polymer (p) by the method described in paragraphs

[0022] to

[0027] of JP-A-2022-104084.

[0040] The average particle size of the pigment-containing polymer particles or pigment-containing crosslinked polymer particles in the pigment dispersion is preferably 60 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, and even more preferably 90 nm or more, from the viewpoint of improving the dispersion stability of the pigment, controlling the behavior of droplets, suppressing printing unevenness, and improving solid filling ability, and is preferably 200 nm or less, more preferably 170 nm or less, and even more preferably 130 nm or less. The average particle size is measured by the method described in the Examples.

[0041] The content of the pigment in the ink according to the present invention is preferably 2% by mass or more, more preferably 3% by mass or more, and even more preferably 4% by mass or more from the viewpoint of ensuring the image density of the obtained printed matter, and from the viewpoint of improving the dispersion stability of the pigment, controlling the behavior of the droplets, suppressing printing unevenness, and improving solid filling ability, it is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less, and even more preferably 10% by mass or less. The content of the polymer dispersant or crosslinked polymer dispersant in the ink according to the present invention is preferably 0.5% by mass or more, more preferably 0.8% by mass or more, and even more preferably 1.0% by mass or more, from the viewpoint of improving the dispersion stability of the pigment, controlling the behavior of droplets, suppressing printing unevenness, and improving solid filling ability, and is preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less, from the viewpoint of improving image density. The mass ratio of the content of the polymer dispersant or crosslinked polymer dispersant to the content of the pigment in the ink according to the present invention [(polymer dispersant or crosslinked polymer dispersant) / pigment] is, from the viewpoints of improving the dispersion stability of the pigment, controlling the behavior of the droplets, suppressing printing unevenness, and improving solid filling ability, preferably 0.1 or more, more preferably 0.2 or more, and even more preferably 0.3 or more, and from the viewpoints of ensuring the image density of the obtained printed matter, is preferably 0.9 or less, more preferably 0.7 or less, and even more preferably 0.5 or less.

[0042] [Water-soluble organic solvent] The ink used in the printing method of the present invention preferably contains a water-soluble organic solvent from the viewpoints of suppressing drying of the ink in the inkjet head nozzles, controlling the behavior of droplets, suppressing printing unevenness, and improving solid printing ability. The water-soluble organic solvent is preferably, for example, at least one selected from the group consisting of polyhydric alcohols, glycol ethers, and cyclic amide compounds.

[0043] Preferred examples of polyhydric alcohols include ethylene glycol, propylene glycol, 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, diethylene glycol, polyethylene glycol, dipropylene glycol, polypropylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 3-methyl-1,3-butanediol, 1,5-pentanediol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol, 1,2,4-butanetriol, 1,2,3-butanetriol, and petriol. Among these, the polyhydric alcohol is preferably one or more selected from the group consisting of alkanediols and alkanetriols, more preferably an alkanediol, even more preferably a 1,2-alkanediol, still more preferably one or more selected from the group consisting of 1,2-butanediol and propylene glycol, and even more preferably propylene glycol.

[0044] Preferred examples of the glycol ether include one or more selected from the group consisting of alkylene glycol monoalkyl ethers and alkylene glycol dialkyl ethers. Preferred examples of the alkylene glycol monoalkyl ether include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol monophenyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, diethylene glycol monohexyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monoethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol monomethyl ether, and dipropylene glycol monoethyl ether. Preferred examples of the alkylene glycol dialkyl ether include ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, diethylene glycol dibutyl ether, diethylene glycol butyl methyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol dibutyl ether, triethylene glycol butyl methyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol dibutyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, dipropylene glycol dimethyl ether, and dipropylene glycol diethyl ether. Among these, the glycol ether is more preferably an alkylene glycol monoalkyl ether, even more preferably one or more selected from the group consisting of diethylene glycol monopropyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, and diethylene glycol monoisobutyl ether, and even more preferably diethylene glycol monoisobutyl ether.

[0045] Preferred examples of the cyclic amide compound include 2-pyrrolidone and 2-methylpyrrolidone. Among these, from the viewpoints of suppressing drying of the ink in the inkjet head nozzle, controlling the behavior of droplets, suppressing printing unevenness, and improving solid filling ability, it is more preferable to use one or more water-soluble organic solvents selected from the group consisting of polyhydric alcohols and glycol ethers, and it is even more preferable to use a combination of one or more polyhydric alcohols and one or more glycol ethers.

[0046] The content of the water-soluble organic solvent in the ink used in the printing method of the present invention is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, from the viewpoints of suppressing drying of the ink in the inkjet head nozzles, controlling the behavior of the droplets, suppressing printing unevenness, and improving solid filling ability, and from the same viewpoints as above, it is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less.

[0047] [Surfactant] The ink used in the printing method of the present invention preferably contains a surfactant from the viewpoints of adjusting the gas-liquid interfacial energy of the ink to control the behavior of droplets and suppressing printing unevenness, and from the viewpoints of improving solid filling ability. Examples of the surfactant include nonionic surfactants, anionic surfactants, and amphoteric surfactants, with nonionic surfactants being preferred. Examples of the nonionic surfactant include acetylene-based surfactants, polyoxyalkylene alkyl ether-type surfactants, polyhydric alcohol-type surfactants, fatty acid alkanolamides, silicone-based surfactants, and fluorine-based surfactants. Among these, the surfactant is preferably one or more selected from the group consisting of acetylene-based surfactants, polyoxyalkylene alkyl ether-based surfactants, and silicone-based surfactants. Among these, it is more preferable to contain one or more selected from the group consisting of acetylene-based surfactants and silicone-based surfactants.

[0048] Preferred examples of the acetylene surfactant include 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol, 3,6-dimethyl-4-octyne-3,6-diol, 3,5-dimethyl-1-hexyne-3-ol, 2,4-dimethyl-5-hexyne-3-ol, and ethylene oxide adducts thereof, with 2,4,7,9-tetramethyl-5-decyne-4,7-diol being more preferred.

[0049] The HLB value of the acetylene-based surfactant is preferably 2 or more, more preferably 2.5 or more, and even more preferably 3 or more, from the viewpoint of adjusting the gas-liquid interfacial energy of the ink to control the behavior of droplets and suppress printing unevenness, and from the viewpoint of improving solid filling ability, and from the same viewpoints as above, it is preferably 18 or less, more preferably 14 or less, even more preferably 10 or less, and even more preferably 7 or less. In the present invention, the HLB value is a value indicating the affinity of a surfactant to water and oil as a hydrophilic-lipophilic balance, and can be calculated by the following formula according to the Griffin method. The catalog value of each product can also be referred to. HLB value = 20 x [(total formula weight of hydrophilic groups contained in surfactant) / (molecular weight of surfactant)] Examples of the hydrophilic group contained in the surfactant include a hydroxy group and an ethyleneoxy group.

[0050] Commercially available acetylene surfactants include Surfynol 104PG50 (a 50% by mass solution of 2,4,7,9-tetramethyl-5-decyne-4,7-diol in propylene glycol, HLB value: 4 (catalog value)), Surfynol 440 (a 3.5 mol adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol with ethylene oxide, HLB value: 8 (catalog value)), Surfynol 465 (a 10 mol adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol with ethylene oxide, HLB value: 13 (catalog value)), and Surfynol DF110D (a 32% by mass solution of 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol in dipropylene glycol, HLB value: 3 (catalog value)) manufactured by Nissin Chemical Industry Co., Ltd.

[0051] As the silicone surfactant, a polyether-modified silicone surfactant is preferred. Suitable examples of the polyether group in the polyether-modified silicone surfactant include polyethyleneoxy groups, polypropyleneoxy groups, and polyalkyleneoxy groups in which ethyleneoxy groups (EO) and propyleneoxy groups (trimethyleneoxy groups or propane-1,2-diyloxy groups; PO) are added in a block or random manner. Compounds in which polyether groups are grafted onto a silicone main chain, and compounds in which polyether groups are bonded in a block form to both ends of a silicone main chain can be used.

[0052] The HLB value of the polyether-modified silicone surfactant is, from the viewpoint of adjusting the gas-liquid interfacial energy of the ink to control the behavior of droplets and suppress printing unevenness, and from the viewpoint of improving solid filling ability, preferably 18 or less, more preferably 17 or less, even more preferably 16 or less, and is preferably 2 or more, more preferably 3 or more, even more preferably 4 or more, still more preferably 6 or more, still more preferably 8 or more, and even more preferably 10 or more.

[0053] Specific examples of polyether-modified silicone surfactants include PEG-3 dimethicone, PEG-9 dimethicone, PEG-9 methyl ether dimethicone, PEG-10 dimethicone, PEG-11 methyl ether dimethicone, PEG / PPG-20 / 22 butyl ether dimethicone, PEG-32 methyl ether dimethicone, PEG-9 polydimethylsiloxyethyl dimethicone, lauryl PEG-9 polydimethylsiloxyethyl dimethicone, and the like. Commercially available polyether-modified silicone surfactants include the KF series manufactured by Shin-Etsu Chemical Co., Ltd., Silface SAG005 manufactured by Nissin Chemical Industry Co., Ltd., and BYK-348 manufactured by BYK Japan K.K.

[0054] The content of acetylene-based surfactant in the ink used in the printing method of the present invention is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and even more preferably 0.4% by mass or more, from the viewpoints of adjusting the gas-liquid interfacial energy of the ink to control the behavior of droplets and suppress printing unevenness, and of improving solid filling ability, and from the same viewpoints as above, it is preferably 1% by mass or less, more preferably 0.8% by mass or less, and even more preferably 0.7% by mass or less.

[0055] The content of the polyether-modified silicone surfactant in the ink used in the printing method of the present invention is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and still more preferably 0.4% by mass or more, from the viewpoints of adjusting the gas-liquid interfacial energy of the ink to control the behavior of the droplets and suppress printing unevenness, and of improving solid filling ability, and from the same viewpoints as above, it is preferably 1% by mass or less, more preferably 0.8% by mass or less, and even more preferably 0.7% by mass or less.

[0056] In the ink used in the printing method of the present invention, it is preferable to use an acetylene-based surfactant and a polyether-modified silicone-based surfactant in combination. When an acetylene-based surfactant and a polyether-modified silicone-based surfactant are used in combination, the mass ratio of the acetylene-based surfactant content to the polyether-modified silicone-based surfactant content [acetylene-based surfactant / polyether-modified silicone-based surfactant] is, from the viewpoint of adjusting the gas-liquid interfacial energy of the ink to control the behavior of the droplets and suppressing printing unevenness, and from the viewpoint of improving solid filling ability, preferably 0.5 or more, more preferably 0.6 or more, even more preferably 0.7 or more, and is preferably 1.5 or less, more preferably 1.3 or less, even more preferably 1.2 or less, and even more preferably 1.0 or less.

[0057] The ink used in the printing method of the present invention may further contain various additives such as fixing resins, humectants, wetting agents, wetting / penetrating agents, viscosity modifiers, defoamers, preservatives, antifungal agents, rust inhibitors, etc. The ink can be produced by mixing a pigment dispersion, a water-soluble organic solvent, and, if necessary, various additives such as water and a surfactant.

[0058] The average particle size of the pigment-containing polymer particles or pigment-containing crosslinked polymer particles in the ink used in the printing method of the present invention is preferably 60 nm or more, more preferably 70 nm or more, even more preferably 80 nm or more, still more preferably 90 nm or more, and is preferably 200 nm or less, more preferably 170 nm or less, even more preferably 150 nm or less, from the viewpoint of improving the dispersion stability of the pigment, controlling the behavior of droplets, suppressing printing unevenness, and improving solid filling. The average particle size is measured by the method described in the Examples. The viscosity of the ink used in the printing method of the present invention at 32°C is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, even more preferably 4 mPa·s or more, and is preferably 12 mPa·s or less, more preferably 9 mPa·s or less, even more preferably 7 mPa·s or less, from the viewpoints of controlling the droplet rate, suppressing printing unevenness, and improving solid filling. The viscosity is measured by the method described in the Examples. The static surface tension of the ink used in the printing method of the present invention is preferably 22 mN / m or more, more preferably 24 mN / m or more, even more preferably 25 mN / m or more, and is preferably 45 mN / m or less, more preferably 40 mN / m or less, even more preferably 35 mN / m or less, from the viewpoints of controlling the droplet separation rate, suppressing printing unevenness, and improving solid filling. The static surface tension is measured by the method described in the Examples. The pH of the ink used in the printing method of the present invention is preferably 7.0 or more, more preferably 7.2 or more, and even more preferably 7.5 or more. From the viewpoint of member resistance and skin irritation, the pH is preferably 11 or less, more preferably 10 or less, and even more preferably 9.5 or less. The pH is measured by the method described in the Examples.

[0059] (drying process) From the viewpoint of suppressing printing unevenness and improving solid filling, it is preferable that the printing method of the present invention further includes a step of maintaining the printing medium, on which the ink has been applied and an image has been formed, in a high-temperature environment during and / or after the image forming step to promote removal of the ink vehicle, the main component of which is water, from the ink coating on the printing medium, i.e., a drying step. The temperature at which the printing medium is held, i.e., the drying temperature, is preferably 30°C or higher, more preferably 35°C or higher, and even more preferably 40°C or higher, from the viewpoint of promoting removal of the ink vehicle and suppressing printing unevenness and improving solid filling, and is preferably 60°C or lower, more preferably 55°C or lower, and even more preferably 50°C or lower, from the viewpoint of suppressing thermal denaturation of the printing medium.

[0060] Preferred means for maintaining the print medium in a high-temperature environment include a method in which gas adjusted to the desired temperature is blown onto the print medium during and / or after the image formation process, a method in which the print medium during and / or after the image formation process is passed through a gas atmosphere adjusted to the desired temperature, a method in which the print medium during and / or after the image formation process is irradiated with an infrared heater, and a method in which the print medium during and / or after the image formation process is heated with a platen heater. Among these, the means for maintaining the printing medium in a high-temperature environment is preferably one or more selected from the group consisting of a method of blowing gas, the temperature of which is adjusted so that the printing medium reaches a drying temperature, onto the printing medium during (A1) the image formation process and / or after (A2) the image formation process, and a method of heating the printing medium during (B1) the image formation process and / or after (B2) the image formation process, using a platen heater whose temperature is adjusted so that the printing medium reaches a drying temperature; more preferably, one or more selected from the group consisting of a method of blowing gas, the temperature of which is adjusted so that the printing medium reaches a drying temperature, onto the printing medium during (A2) the image formation process, and a method of heating the printing medium during (B1) the image formation process and / or after (B2) the image formation process, using a platen heater whose temperature is adjusted so that the printing medium reaches a drying temperature; and even more preferably, a combination of the method of blowing gas adjusted to a desired temperature onto the printing medium during (A2) the image formation process and the method of heating the printing medium during (B1) the image formation process and (B2) the image formation process, using a platen heater. In this case, the speed of the hot air when blowing gas whose temperature has been adjusted so that the printing medium reaches the drying temperature onto the printing medium is preferably 5 m / s or more, more preferably 7 m / s or more, even more preferably 8 m / s or more, from the viewpoint of suppressing printing unevenness and improving solid filling, and is preferably 15 m / s or less, more preferably 13 m / s or less, and even more preferably 12 m / s or less. EXAMPLES

[0061] In the following Production Examples, Examples and Comparative Examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. The methods for measuring each physical property are as follows.

[0062] (1) Measurement of weight average molecular weight of water-insoluble polymer (p) The measurement was performed by gel permeation chromatography under the following conditions. GPC equipment: Tosoh Corporation "HLC-8320GPC" Columns: Tosoh Corporation's "TSKgel SuperAWM-H", "TSKgel SuperAW3000", and "TSKgel guardcolum Super AW-H" Eluent: N,N-dimethylformamide with phosphoric acid and lithium bromide dissolved at concentrations of 60mmol / L and 50mmol / L, respectively. Flow rate: 0.5mL / min Standard material: Monodisperse polystyrene kit with known molecular weight [PStQuick b (F-550, F-80, F-10, F-1, a-1000), PStQuick C (F-288, F-40, F-4, a-5000, a-500)] (manufactured by Tosoh Corporation) Measurement sample: 0.1 g of the polymer to be measured was mixed with 10 mL of the eluent in a glass vial, stirred with a magnetic stirrer at 25°C for 10 hours, and filtered with a syringe filter "DISMIC-13HP" (PTFE, 0.2 μm, Advantec Co., Ltd.) for use.

[0063] (2) Measurement of the acid value of water-insoluble polymer (p) The water-insoluble polymer (p) was dissolved in a titration solvent of toluene and acetone (volume ratio = 2:1) in an automatic potentiometric titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with 0.1N potassium hydroxide / ethanol solution by potentiometric titration. The inflection point on the titration curve was taken as the end point. The acid value (mgKOH / g) was calculated from the titration amount of potassium hydroxide solution up to the end point.

[0064] (3) Measurement of solids concentration 10.0 g of sodium sulfate, which had been kept constant in a desiccator, was weighed out into a 30 mL polypropylene container (φ=40 mm, height=30 mm), and about 1.0 g of the sample was added and mixed, then accurately weighed, and the mixture was kept at 105°C for 2 hours to remove volatile matter, and then left in the desiccator for 15 minutes, after which the mass was measured. The mass of the sample after removing the volatile matter was taken as the solid content, and divided by the mass of the sample added to obtain the solid content concentration (%).

[0065] (4) Measurement of the average particle size of pigment-containing polymer particles in a pigment water dispersion or ink Using a laser particle analysis system "ELS-8000" (Otsuka Electronics Co., Ltd.), the particle size was measured by dynamic light scattering, and cumulant analysis was performed. The obtained cumulant average particle size was taken as the average particle size of the pigment-containing polymer particles. The measurement sample had a particle concentration of 5 × 10 -3 A dispersion liquid diluted with water to a mass % (solid content conversion) was used. The measurement conditions were a temperature of 25°C, an angle between the incident light and the detector of 90°, and 100 cumulative measurements. The refractive index of water (1.333) was entered as the refractive index of the dispersion solvent.

[0066] (5) Measurement of ink viscosity The viscosity of the ink at 32° C. was measured using an E-type viscometer “TV-25” (manufactured by Toki Sangyo Co., Ltd., standard cone rotor 1°34′×R24, rotation speed 50 rpm).

[0067] (6) Measurement of the static surface tension of the ink Using a surface tensiometer "CBVP-Z" (manufactured by Kyowa Interface Science Co., Ltd.), a platinum plate was immersed in a cylindrical polyethylene container (diameter 3.6 cm x depth 1.2 cm) containing 5 g of water-based ink, and the static surface tension of the ink was measured at 20°C.

[0068] (7) Measuring the pH of the ink The pH of the ink at 25° C. was measured using a tabletop pH meter “F-71” (manufactured by Horiba, Ltd.) that uses a pH electrode “6337-10D” (manufactured by Horiba, Ltd.).

[0069] (8) Ink dot diameter After applying ink by inkjet printing, the printing media used in the examples were observed with a digital microscope "RH-2000" (manufactured by Hirox Corporation), and 10 ink dots each of the first and second colors were observed, and the average value was taken as the diameter of the ink dots.

[0070] (Production of Water-Insoluble Polymer (p)) Manufacturing Example 1-1 A monomer mixture was prepared by mixing 31 parts of acrylic acid and 69 parts of styrene. 10 parts of methyl ethyl ketone (hereinafter referred to as "MEK"), 0.2 parts of 2-mercaptoethanol as a polymerization chain transfer agent, and 10% of the monomer mixture were mixed in a reaction vessel, and the inside of the vessel was thoroughly replaced with nitrogen gas. Separately, a mixture of the remainder of the monomer mixture (90% of the monomer mixture), 0.13 parts of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) ("V-65" manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) as a radical polymerization initiator was placed in a dropping funnel. Next, under a nitrogen atmosphere, the monomer mixture in the reaction vessel was heated to 65° C. while stirring, and the mixture in the dropping funnel was then dropped over 3 hours. After 2 hours had passed since the end of the dropping while maintaining the temperature at 65° C., a solution in which 0.1 part of the polymerization initiator was dissolved in 2 parts of MEK was added, and the mixture was further aged at 65° C. for 2 hours and at 70° C. for 2 hours, and then dried under reduced pressure to obtain a water-insoluble polymer (p-1) (weight average molecular weight: 19,000, acid value: 240 mgKOH / g).

[0071] (Production of Pigment Water Dispersion) Manufacturing Example 2-1 100 parts of the water-insoluble polymer (p-1) obtained in Production Example 1-1 was mixed with 78.6 parts of MEK, and 41.2 parts of a 5N aqueous sodium hydroxide solution (sodium hydroxide content: 16.9%) was added as a neutralizing agent to neutralize (neutralization degree: 40 mol%). 800 parts of ion-exchanged water was then added, and 100 parts of cyan pigment (CI Pigment Blue 15:3) was added thereto, and the mixture was stirred for 3 hours at 20°C using a disperser (Asada Iron Works Co., Ltd.'s "Ultra Disperser") with the disperser blade rotating at 6,000 rpm. Next, 124 parts of ion-exchanged water was added, and the mixture was dispersed 15 times using a Microfluidizer (Microfluidics Co., Ltd., product name) at a pressure of 150 MPa to obtain a pigment dispersion. The obtained pigment dispersion was placed in a 500 mL angle rotor and centrifuged at 3,660 rpm for 20 minutes using a high-speed cooled centrifuge (Hitachi Koki Co., Ltd., "himac CR22G", set temperature 20°C). The liquid phase was then recovered and filtered through a 5 μm membrane filter (Sartorius, "Minisart") to obtain a cyan pigment aqueous dispersion D1 containing cyan pigment-containing polymer particles (solid concentration: 25%, cyan pigment content: 19%, polymer content: 6%, average particle size of cyan pigment-containing polymer particles: 105 nm).

[0072] Manufacturing Examples 2-2 to 2-3 In the same manner as in Production Example 2-1, except that the cyan pigment was changed to a magenta pigment (CI Pigment Red 122) and a yellow pigment (CI Pigment Yellow 74), respectively, a magenta pigment water dispersion D2 (solid concentration: 25%, magenta pigment content: 19%, polymer content: 6%, average particle size of magenta pigment-containing polymer particles: 105 nm) and a yellow pigment water dispersion D3 (solid concentration: 25%, yellow pigment content: 19%, polymer content: 6%, average particle size of yellow pigment-containing polymer particles: 120 nm) were obtained.

[0073] (Water-based ink manufacturing) Manufacturing Example 3-1 105.26 parts of the cyan pigment aqueous dispersion D1 (solid concentration 25%) obtained in Production Example 2-1, 100 parts of propylene glycol (boiling point 188 ° C), 20 parts of diethylene glycol monoisobutyl ether (boiling point 230 ° C), 5 parts of an acetylene glycol surfactant (Nissin Chemical Industry Co., Ltd.'s "Surfynol 104PG50", active content 50%), 3.25 parts of a silicone surfactant (Shin-Etsu Chemical Co., Ltd.'s "KF-6011", polyether modified silicone), and 266.49 parts of ion-exchanged water were added and mixed. The resulting mixture was filtered with a filter "Mini Sart Syringe Filter" (Sartorius, pore size: 5 μm, material: cellulose acetate) to obtain a cyan aqueous ink C-1. The physical properties of the obtained cyan aqueous ink C-1 are shown in Table 1.

[0074] Manufacturing Examples 3-2 to 3-3 A magenta water-based ink M-1 and a yellow water-based ink Y-1 were obtained in the same manner as in Production Example 3-1, except that the cyan pigment water dispersion D1 was replaced with the magenta pigment water dispersion D2 and the yellow pigment water dispersion D3, respectively.

[0075] [Table 1]

[0076] Example 1 A corona discharge treated PET film ("Taiko Polyester Film FE2001", 25 μm thick, manufactured by Futamura Chemical Co., Ltd.) was prepared as a printing medium. In an environment with a temperature of 25±1°C and a relative humidity of 30±5%, a printing evaluation device (manufactured by Tritec Corporation) equipped with an inkjet printing line head (Kyocera Corporation "KJ4b-1200", piezo type, the number of nozzles per inch in the longitudinal direction of the nozzle row of the inkjet head is 1200, the standard value of the applied voltage of the piezo element is 26V) was filled with cyan water-based ink C-1, magenta water-based ink M-1, and yellow water-based ink Y-1, and each ink was printed on the printing medium in the order of the line head filled with the cyan water-based ink, the line head filled with the magenta water-based ink, and the line head filled with the yellow water-based ink. The distance between the line heads filled with each color ink was set to 55 mm. A cyan water-based ink C-1 was filled into the line head of the print evaluation device as the first color ink to be applied to the print medium, and 20 specific nozzles were observed using the print evaluation device's discharge observation mode (an inkjet discharge observation device for observing ink discharged based on a single discharge command for discharging ink from the inkjet head (a device that adjusts the interval of strobe light emission so that the ink discharged from the inkjet head appears to be stationary, and captures the ink that appears to be stationary with a video camera (frame rate: 24 fps to 60 fps))). When conditions were set such that ink droplets were observed within a distance range of 1 mm to 2 mm (imaging magnification: 180x) from the nozzle plate of the inkjet head, the temperature inside the inkjet head was set to 45°C, and the applied voltage of the piezoelectric element of the inkjet head was set to 35V, which were conditions such that the droplet separation rate calculated by the following formula would be 100% from the number of nozzles from which droplet separation was confirmed for ink droplets present within that distance range. Droplet rate (%) = (number of nozzles with confirmed droplets / 20) x 100 A line head filled with magenta water-based ink M-1 as the second color ink to be applied to the printing medium was set under conditions such that the droplet rate calculated in the same manner as above would be 0%, with the temperature inside the inkjet head set to 35°C and the applied voltage of the piezoelectric element of the inkjet head set to 26 V. Other driving conditions for the inkjet head were set as follows: driving frequency 16 kHz, ejection droplet volume 2.5 pl, number of pre-ejection flushings 200 shots, and negative pressure -4.0 kPa. An A3-sized film heater (manufactured by Kawai Electric Manufacturing Co., Ltd.) was fixed to the transport table of the printing evaluation device so that the printing medium could be heated from the side opposite the side of the printing medium facing the inkjet head, and the printing medium was set on the transport table so that the longitudinal direction of the printing medium was the same as the transport direction of the printing medium. At this time, the distance between the film heater and the printing medium was set to 0.25 mm, the distance between the inkjet head and the printing medium was set to 1.0 mm, and the surface temperature of the film heater was set to 45°C so that the temperature of the surface of the printing medium became 40°C. Next, a print command was transferred to the print evaluation device to superimpose solid images with a print duty of 50% for each color in the order of cyan water-based ink C-1 and magenta water-based ink M-1 at a print medium conveying speed of 20 m / min, the temperature inside the inkjet head set above, and the applied voltage of the piezoelectric element of the inkjet head, and the cyan water-based ink C-1 and magenta water-based ink M-1 were applied to the print medium by the inkjet recording method to form a solid image of the secondary color, and the solid image was dried by blowing hot air at 45 ° C. at a speed of 10 m / s for 20 seconds on the print medium on which the solid image of the secondary color was formed, to obtain a print for evaluation of solid filling. At this time, the diameter of the ink dots of the first color was 25 μm, and the diameter of the ink dots of the second color was 40 μm. Using the obtained print, the solid filling was evaluated by the method shown below. In the production of the print for evaluating the solid filling property described above, a secondary color solid image was formed in the same manner as above, except that instead of the print command for overlapping solid images with a print duty of 50% for each color, a print command for overlapping solid images with a print duty of 100% for each color was used, and a print for evaluating print unevenness was obtained. The print obtained was used to evaluate print unevenness by the method described below.

[0077] Examples 2 to 7, Comparative Examples 1 to 3 Inkjet printing was carried out in the same manner as in Example 1, except that the inks shown in Table 2 were combined and the temperature inside the inkjet head and the applied voltage to the piezoelectric element of the inkjet head were adjusted using the same method as in Example 1 so that the droplet rates of the inks ejected for the first and second colors were the values ​​shown in Table 2, and images of two colors were formed, and printed matter for evaluating solid filling and printed matter for evaluating print unevenness were obtained. Solid filling and print unevenness were evaluated by the methods shown below. In addition, in Comparative Example 2, since it was difficult to form a solid image due to droplets of the second color ink, it was not possible to evaluate printing unevenness.

[0078] Example 8 Inkjet printing was performed in the same manner as in Example 1, except that the ink filling portion was changed so that magenta water-based ink was printed as the first color and cyan water-based ink was printed as the second color, and the temperature inside the inkjet head and the ejection force of the inkjet head were adjusted in the same manner as in Example 1 so that the droplet rates of the inks ejected for the first and second colors were the values ​​shown in Table 2, and images of two colors were formed, and a printed matter for evaluating solid filling and a printed matter for evaluating print unevenness were obtained. Solid filling and print unevenness were evaluated by the methods described below.

[0079] <Solid coverage> The images formed on the prints for evaluating solid filling obtained in the Examples and Comparative Examples were visually observed, and the solid filling of the secondary color was evaluated according to the following evaluation criteria. An evaluation score of 5 indicates excellent solid filling of the secondary color. An evaluation score of 1 indicates poor solid filling of the secondary color. (Evaluation Criteria) 5: No streaks and particularly good solid filling. 4: Very slight streaks are visible, but the fill is good. 3: Some streaking is visible, but the fill is generally good. 2: There are streaks and some areas where the solid paint has not been filled in. 1: Many streaks are visible and there are areas where the solid color has not been filled in.

[0080] <Printing unevenness> The secondary color solid images formed on the prints for evaluating print unevenness obtained in the Examples and Comparative Examples were visually observed, and mottling (irregular unevenness in ink density) was evaluated according to the following evaluation criteria. An evaluation criterion of 5 indicates that print unevenness is suppressed and print quality is excellent. An evaluation criterion of 1 indicates that print unevenness is significantly observed and print quality is poor. (Evaluation Criteria) 5: No mottling is observed on solid images, and printing unevenness is particularly well suppressed. 4: Slight mottling is observed on solid images, but printing unevenness is well suppressed. 3: Some mottling is observed on solid images, but printing unevenness is generally well controlled. 2: Mottling is observed on solid images in parts of the image formation area, causing printing unevenness. 1: Mottling is observed on solid images in almost all image forming areas, causing printing unevenness.

[0081] [Table 2]

[0082] From Table 2, it can be seen that in Examples 1 to 8, the droplet rate of the ink ejected as the first color is greater than the droplet rate of the ink ejected as the second color and thereafter, and therefore, compared to Comparative Examples 1 to 3, they have superior solid filling performance and also suppress printing unevenness.

Claims

1. An inkjet printing method using two or more inks, comprising the step of applying two or more inks to a printing medium to form an image, under the condition that the droplet rate of the first color of ink ejected is greater than the droplet rate of the second and subsequent colors of ink.

2. The inkjet printing method according to claim 1, wherein the droplet rate of the ink ejected as the first color is 50% or more.

3. The inkjet printing method according to claim 1 or 2, wherein the droplet rate of the ink ejected for the second and subsequent colors is 0% or more and 30% or less.

4. The inkjet printing method according to claim 1 or 2, wherein the inkjet head is a line head type.

5. The inkjet printing method according to claim 1 or 2, wherein the resolution in a direction parallel to the transport direction of the printing medium is 600 dpi or more.

6. The inkjet printing method according to claim 1 or 2, wherein the resolution in a direction perpendicular to the transport direction of the printing medium is 600 dpi or more.

7. The inkjet printing method according to claim 1 or 2, wherein the diameter of the ink dots that land on the printing medium is 20 μm or more.

8. The inkjet printing method according to claim 1 or 2, wherein the transport speed of the printing medium is 10 m / min or more.

9. The inkjet printing method according to claim 1 or 2, wherein the image formed by the above step is an image with two or more secondary colors.

10. The inkjet printing method according to claim 1 or 2, wherein the ink is a water-based ink.

11. The inkjet printing method according to claim 1 or 2, wherein the droplet rate is controlled by adjusting the temperature inside the inkjet head.